CL - Seasons (remote)
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California State University, Bakersfield *
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1609
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Astronomy
Date
Apr 3, 2024
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docx
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Uploaded by crystalized47
Computer Lab – Seasons
(Virtual Lab Remote Edition)
Sun Distance
A common misconception is that the seasons on Earth are due to the varying Earth-
Sun distance. While it is true that the Earth-Sun distance does vary (Earth’s orbit is elliptical)
and the Earth does receive more sunlight when closer to the Sun, this is a small effect and
does not explain the seasons. For one thing, if this was the cause of the seasons, people in
the northern and southern hemispheres should have summer and winter at the same times,
but we have summer in the northern hemisphere when people in the southern hemisphere
have winter. Let’s figure out what days in the coming year the Earth will be closest to and
furthest from the Sun. Launch the Voyager application by navigating a browser to csub.apporto.com, signing
in, and running the Voyager 4 app. If the Chart window appears a bit small on screen, click
the square
window zoom button in the title bar to expand the Sky Chart
window so it fills the screen. Set the Time Step in the Time Panel to 7 days, click once on the Sun
to open its Info Panel (if you don't see the Sun, either use the scroll bars to
find it or select the Center/Planets/Sun menu), and find where the "Distance" to the Sun is
listed in astronomical units (AU). Advance the time in single steps (the "|>" button just to
the right of the Start button) and watch the distance value. Not
seeing this? Make sure you opened the Info Panel for the Sun and not some other object.
With each step of time, you should see the Distance either increasing or decreasing.
Advance through a whole year, noting the largest and smallest values of this Earth-Sun
distance. Record below the largest and smallest distances you saw and a date when that
value occurred.
Furthest distance from Sun:
Date: July 12, 2033
Distance: 1.01655 AU
Closest distance to Sun:
Date: Nov 29, 2033
Distance: 0.98645 AU
1
Surprisingly, the Earth is furthest from the Sun on about July 3 each year and closest to
the Sun around January 4 (these dates can vary slightly from year to year because of
precession and leap years).
The furthest point in an orbit from the Sun is called aphelion, the closest point is called
perihelion. If you repeated this for any other year, you would get basically the same results.
It is winter (for us) when Earth is closest to the Sun! The Earth-Sun distance does not explain
the seasons, although it does make our winters and summers slightly more mild than those
in the southern hemisphere.
Eratosthenes Experiment
Select the Chart/Set Location… menu, type in Syene for the Location Name. Use the
tab key to advance from field to field, enter longitude 32° 31' E, latitude 23° 26' N, 0 for
elevation, and 2.0 for the Time Zone. Click OK.
Select the Chart/Set Time… menu, change the date to June 21 of this year, turn off
Automatic Daylight Savings Time, and click OK. If the Sun's Info Panel is already open,
click the Center button, if not select the Center/Planets/Sun menu. Click the Visibility tab in
the Info Panel and record the Transit Time and Altitude:
Syene:
Transit Time = 11:52 AM
Altitude = 90°
Altitude measures how high an object appears currently in the sky, an altitude of 0°
means an object is along the horizon, 90° means an object straight overhead. The altitude at
transit should say 90° (if not, go back and re-check your work), the Sun passes through
zenith at Syene at the summer solstice. Although this occurs at local noon, the time is not
12:00 PM at Syene because of time zones.
Again select the Chart/Set Location… menu, click the "+" button below and left of the
globe map repeatedly until the name "Aswan" appears on the map. The ⊗
marker shows
your current viewing location – Syene. The city Syene does not exist anymore, it was located
near the modern city of Aswan, site of the famous Aswan Dam along the Nile river.
Eratosthenes was a Greek astronomer who lived around 200 BC in the city of
Alexandria. He knew that on the first day of summer (summer solstice) the Sun was directly
overhead (at zenith) at the city of Syene at noon as we just saw. The city of Alexandria is north of Syene, scroll the map north (up) until
you see Alexandria. Click on the white dot for Alexandria and click OK. 2
Click on the Sun's icon in the Chart window to update the Sun's Info Panel display. If
you are still seeing the same times you had for Syene, either you did not change locations
when in the Set Location window or you have not clicked on the Sun to get its updated Info
Panel. Record the time and altitude of the Sun's transit (noon) for Alexandria:
Alexandria:
Transit Time = 12:03 PM
Altitude = 82°
Although the Sun would pass directly overhead of Syene on the summer solstice, it
was south of the zenith at noon in Alexandria on the same day. Eratosthenes realized that
this was due to the curvature of the Earth and could be used to measure the size of Earth
(see the section in chapter 2 of our textbook).
How many degrees away from zenith is the Sun as seen in Alexandria at noon on the
summer solstice? You calculate that by taking the difference between 90° and your just-
determined altitude.
Angle from zenith = 90° – 82° = 7.2°
Eratosthenes had no computer, he determined this angle by measuring the angle of a
shadow cast by a vertical object. We can use this result to calculate the radius of Earth just as Eratosthenes did as
explained in the text. The distance from Syene to Alexandria is about 865 km.
Equating Ratios: 7.2° = 8,000
Doing the algebra to solve for the (circumference of Earth) gives the following. Finish
the calculation by putting in your value for the (angle from zenith) from above into the
brackets beneath the 360°:
(circumference of Earth) = 865 km = ______________ km
The circumference of an object is (2π) times the radius, so the radius is the
circumference divided by (2π). [π = 3.14 if your calculator doesn’t have it]
Radius of Earth = = 6371 km
This result should be fairly close to the textbook value of 6378 km. Didn’t get very
close? The most common math error here is doing [ ÷ 2 × π ] instead of [ ÷ ( 2 × π ) ] on your
calculator. We could have obtained even better results if we had measured the altitude at
Alexandria more accurately.
3
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[Answers to most questions can be found at the end of the lab instructions.]
1.
Did Eratosthenes know that Earth was round?
no
2.
Why did we have to set the date to June 21?
Because only at noon on that day did Eratosthenes know exactly where the Sun was in
Syene’s sky (straight overhead, zenith) to compare with the Sun’s position in Alexandria’s
sky.
Eastern Horizon
Select the File/Open Recent Settings/Startup menu, this puts us back to the startup
configuration – viewing from Bakersfield at the current time. Select 90° from the zoom pop-
up menu, select the Center/Direction/East menu. Select the
Display/Sky and Horizon… menu, click to turn on Cardinal
Points, then click OK. Set the Time Step to 2 min.
Start the animation and watch how the stars
move, stop when you see the Sun reaching the
horizon.
3. Which direction do the stars move as they
rise in the east relative to the horizon? N
East
S
Multiple choice:
(a) (b) (c)
(a) Stars rise angling southward
(b) Stars rise moving vertically
(c) Stars rise angling northward
4. Does the Sun rise north (left)
or south (right)
of the due east direction?
5. What you are seeing is for Bakersfield, looking east, for the current time of the year.
Would your answers be the same for a different time of year?
(a) Does the answer to Q3 vary depending on the time of year?
yes
no
(b) Does the answer to Q4 vary depending on the time of year?
yes
no
Continue the animation. You will see the Moon and planets rising once a day like the
Sun and stars. Like the Sun, the planets can vary their north/south positions, sometimes
rising north and sometimes south of due east. But none of them vary their north/south
positions in the same one-year time frame as the Sun. The moon, for example, takes just one
month to vary from north to south and back. If you look closely where the Moon rises on
screen, you should see a large shift by the next moonrise.
4
Western Horizon
Click "Stop" and then the "Now" button in the Time Panel. Click on the
triangle next to the clock in the Time Panel and select Sunset from that pop-
up menu. Select the Center/ Planets/Sun menu, click "Lock" in the
Sun's Info Panel, close the Info Panel. Zoom to 120°, can you see
Venus (white diamond) on screen? If not, temporarily zoom out
and/or close the Sun’s Info Panel.
We are now looking west at the setting Sun. If Venus appears
above the Sun this evening, then Venus is an "evening star" – visible in the evening sky after
the Sun has set. If Venus appears below the Sun (in the green), then it is a "morning star",
not visible in the evening but it will be visible in the morning before the Sun rises.
6. Is Venus currently an evening or a morning star?
morning star
Set the Time Step to 1 day and animate. Watch how Venus shifts from above to below
the Sun and back. Remember that we are using a time step of 1 day, we are looking at
successive snapshots of the sky taken every evening at the same time.
Also watch what the Sun does, it not only moves north and south but also gets higher
and lower in the sky. Overall, the Sun traces out a figure-eight type path that's called the
"analemma". This is due to the tilt of Earth as well as the elliptical orbit of Earth.
If you're like me, you've been staring at the screen so fascinated that ten years have
passed. Maybe you're not like me.
Analemma
Select the File/Open Settings… menu, open the "110 Settings" folder, [first time today
so you must navigate through the folders; This PC:, Program Files, Carina Software,
Voyager 4, 110 Settings] and open the file "Analemma". Read and close the text window.
Start the animation. The Sun appears to move north and south (up and down on
screen) because Earth is tilted. Winter solstice is when the Sun appears at its most southern
position, summer solstice at its most northern.
That the Sun also moves left and right on screen (east and west in the sky) is due to the
non-circularity (ellipticity) of the Earth's orbit. This causes the days to vary in length, the
Sun does not return to the same east-west location every 24 hours. For half the year the days
are longer and the Sun after every 24-hour period appears further east in the sky, then the
Sun drifts westward for the other half of the year. The combination of these north-south and
east-west motions give rise to the figure-8 pattern called the analemma.
5
Select the Window/Location Panel menu and continue
the animation. Watch how the pattern of sunlight changes
within the Location Panel.
That animation was with 1-day time steps. Change the
Time Step to 5 min and animate again. The pattern of light
and dark in the Location Panel slides to the left (west)
because the Earth is rotating eastward. Why is the nighttime region so oddly shaped in the
Location Panel? Because of two reasons. First, because the spherical half-lit Earth has been
forced onto a rectangular map – this causes the polar regions to become greatly stretched
and distorted. Secondly, because the Earth is tilted causing either the northern or southern
hemisphere to get more sunlight – this varies with the season.
7. How long does it take the dark region to make a complete loop in the Location
Panel?
24 hours
Stop the animation. Sunlight and Location
Open the Setting File called "Changing the Location", read and close the text
window. You will need to click the Expand button (
next to
the red close button)
in the Location Panel to see the whole thing. The location marker (
⊗
) in the Location Panel
is currently atop New York City. Click and drag the location marker around in the Location Panel. Try moving it up-
and-down and left-and-right, watch how the corresponding position of the Sun in the sky
changes. Note how the Sun is not visible up in the sky when the location is in the dark areas
(nighttime!).
8. Does the Sun appear higher or lower in the sky from Bakersfield compared to NY at
this time?
Higher, the local time in New York is 3:07 PM, it’s 12:07 PM in Bakersfield.
9. What does that yellow circle in the Location Panel represent?
The Sun! Or, more accurately, the place on the Earth for which the Sun is straight
overhead.
Daylight and Location
6
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Select the File/Open Recent Settings/Startup menu; that returns us to the view from
Bakersfield at the current time. Select the Chart/Set Time… menu and set the date to June 21
of the current year, leave Automatic Daylight Savings Time on. Select the Center/Planets/Sun menu, click on the "Visibility" tab within the Sun's Info
Panel. Fill in the first line of the table below using the data from the Info Panel. The
longitude and latitude are listed following "Bakersfield" near the bottom of the chart
window, these have already been filled in for you in the first
line of the table, the angles have been rounded to the nearest
degree.
You must calculate the "Length of Daylight" using the sunrise and sunset times. This is
most easily done by figuring the hours and minutes from sunrise to noon (12:00) and then
adding the time from noon to sunset. For Bakersfield on June 21, you should have found
that the sunrise was 5:45 AM and sunset was 8:12 PM (the exact times could be different
depending on the year being used), the daylight time was calculated like this:
Time from 5:45 AM to 6:00 AM
15m
Time from 6:00 AM to 12:00 noon
6h
Time from 12:00 noon to 8:12 PM
8h 12m
Total number of hours and minutes
14h 27m
[
If the number of minutes had been >60, I would have reduced the minutes by 60 and
added another hour to the time.
]
[
Warning: Length of Daylight calculation problems often appear on midterm exams!
]
City
Longitude
Latitude
Sunrise
Sunset
Length of Daylight
Bakersfield
119° W
35° N
5:45 AM
8:12 PM
14h 27m
Los Angeles
118° W
34° N
5:45 AM
8:05 PM
14h 20m
Las Vegas
115° W
36° N
5:25 AM
7:57 PM
14h 32m
Walla Walla
118° W
46° N
5:04 AM
8:43 PM
15h 39m
Select the Chart/Set Location… menu, change your viewing location to Los Angeles.
You can do this by zooming in on the globe and clicking on the white dot next to Los
Angeles or by selecting Los Angeles from the city list on the left side of the window. Click
OK.
Usually, the Sun’s Info Panel fails to update after doing a change
of location like this. Click on the Sun in Sky Chart window to update
the Sun’s data (make sure you are still seeing the Sun’s data; it should
look like the picture here). 7
The longitude, latitude, sunrise, and sunset have been entered for you in the table
above. Calculate the length of daylight for Los Angeles.
10. Which city, Los Angeles or Bakersfield, has the longer length of daylight on June
21? Bakersfield by about 7 minutes.
Select the Chart/Set Location… menu again, set your location to Las Vegas (either from
the city list or on the globe, Las Vegas is east (right) of Bakersfield). Again you'll need to
click on the Sun to update the Info Panel. Fill in the rest of the Las Vegas line of the above
table. You should find that daylight in Las Vegas is slightly longer than for Bakersfield, this
is because Vegas is further north than Bakersfield, which was further north than L.A.
11. Why were both
the sunrise and sunset times earlier
for Las Vegas than for
Bakersfield? (Try to answer yourself before reading the answer in the back.)
Because Las Vegas is substantially east of Bakersfield.
Earth rotates from west towards east. That’s why things rise in the east and set in the
west, it’s not that they are moving around the Earth but that Earth is whirling us around.
Las Vegas is east of Bakersfield; they are ‘ahead’ of us and see the Sun rise before we do.
Earth must rotate for an extra 15 minutes or so before we can first see the Sun from
Bakersfield after they’ve seen it in Vegas. People on the east coast are about 3 hours ‘ahead’
of us. The Sun will rise and set as seen from Bakersfield about 3 hours later – at least
compared to cities at about the same latitude (like Raleigh, North Carolina).
Now set your location to Walla Walla, Washington, a city roughly due north of
Bakersfield in the state of Washington. Fill in the Walla Walla line of the table.
12. As we go further north, the daylight gets longer; just how long can it get?
On the summer solstice, all places within the northern Arctic Circle get 24 hours of
daylight.
South of Bakersfield the days will be shorter. On the equator, daylight will last 12
hours – on the equator every day of the year is 12 hours, not just summer solstice. In the
southern hemisphere on June 21, they will be having their shortest day of the year, no
daylight at all within the southern Arctic Circle.
On an equinox, every place on Earth gets 12 hours day and 12 hours night. In the
winter in the northern hemisphere our days are less than 12 hours. The southern
hemisphere is then having summer with days more than 12 hours.
8
Earth’s Seasons
Select the File/Open Recent Settings/Startup menu.
Select the Window/Location Panel menu. Click on the Solar
System tab and change Sun to Earth. Set the Ecliptic
Longitude value to 180°, the Ecliptic Latitude value to 0, and
drag the Distance AU slider down around 0.0001 (so that
Earth half fills the window). Close the Location Panel or drag
it to corner.
Select the Display/Planets and Moons…
menu. For the Earth, leave the Show Planet,
Show Phase, and Surface Image options
turned on. Turn on Show Axis; turn or
leave off all the other options. Click OK.
Turn off stars (because they will be
distracting otherwise) using the third
control button along the right edge of the screen. Use the pop-up control on the Time Panel
to set local time to noon, this should center North America in our view.
Do you see the purple arrow sticking through the Earth? It
enters the Earth at the south pole (in Antarctica) and exits through the
north pole. If not, shrink the size of the Earth being seen using either
the zoom controls or the Location Panel. The purple arrow is the
Earth’s rotation axis, the axis it spins around.
With a Time Step of 1 day, start the animation. Watch how the
purple arrow seems to move in a circle. We are viewing the Earth
from the direction of the Sun, when the purple arrow is pointing
away from our view the whole northern hemisphere is tilted away
from the Sun. Not much sunlight hitting North America at that time.
Stop the animation when the purple arrow is pointing most directly towards you (the
purple arrow will be straight up and down at that time). Record the date you stopped at.
13.
Date: Around June 21.
Change the Time Step to 1 min and animate. This is the Earth at or near the (northern)
summer solstice. Locations within the (northern) arctic circle are getting sunlight for the
whole day (24 hours). All northern locations are getting daylight lasting longer than 12
hours and the Sun appears higher in the sky (more nearly overhead) at noon for these
locations.
Set the Time Step back to 1 day and re-animate. While it looks like the Earth is
“wobbling”, it is really maintaining a constant tilt; that purple arrow is always pointing in
the direction of the North Star (Polaris). The spinning Earth is moving in its orbit around the
Sun, the tilt that was towards the Sun will be away from the Sun six months later.
9
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The Earth is tilted relative to the plane of its orbit by 23.5°. When the Earth is in the
part of the orbit where the northern hemisphere tilts towards the Sun, the northern
hemisphere gets more sunlight and has summer. At that same time, the southern
hemisphere is tilted away from the Sun and has winter. It is the tilt of the Earth that causes
the seasons, not the changing Earth-Sun distance.
Stop the animation around December 22 and you’ll be at winter solstice (for the
northern hemisphere). Advance the time to either March 21 or September 21 and you’ll be at
an equinox. At the equinox, the Earth’s tilt is “sideways”, neither towards nor away from
the Sun. While still tilted, all locations on Earth will get equal lengths of daytime and
nighttime.
I'm confused. If the Earth's rotation axis always points the same direction (towards
Polaris), how can it be swinging alternately towards and away from the Sun? Can you
explain this? While I'm sure you know the answer to this question, you may have had
difficulty putting it into words. Open the Settings File called "Tilt, Orbit, and Seasons".
Start the animation. The Earth is always tilted the same direction (up and right on the
screen) but it is also orbiting the Sun. Around June 21, being tilted towards Polaris is also
being tilted towards the Sun (for the northern hemisphere). Six months later, we're still
tilting towards Polaris but now that is away from the Sun.
14. The direction of Earth's tilt ______ (multiple choice)
(a) changes and repeats every 23h 56m.
(b) changes and repeats every 24h.
(c) changes and repeats every 1 year.
(d) never changes.
Stop the animation. Select the File/Open Recent Settings/Startup menu, select the
Window/Location Panel menu. Look at how the nighttime region is displayed on the
Location Panel map – does that make sense based on the current date and Earth's tilt? Start
the animation (1 day steps) and watch how the daylight/nighttime regions shift in the
Location Panel (if it goes too fast, advance the time using the single-step button on the
Time Panel).
Stop the animation when the boundary between day and night is (close to) a straight
vertical line.
15. On what day(s) of the year does this occur?
March 21 and another around September 22
Change the Time Step to 2 min, animate, and watch the Location Panel map. It should
be clear that every spot on Earth is getting roughly the same length of daylight.
The Earth from the Moon
10
Select the File/Open Recent Settings/Startup menu,
select the Window/Location Panel menu. Click on the Solar
System tab, select the Moon from the pop-up menu, and
move the Distance slider control down to the bottom so that it
says 0. Close the Location Panel.
Select the Window/Planet Panel menu, double-click on
the Earth button, this should both turn on Earth so it can be
seen and center it on the screen. Click the lock symbol next to
Earth, close the Planet Panel. Zoom to 10°.
We are viewing the Earth from the Moon. You might happen to
be looking at just the dark side of Earth (everything will be black), to fix this just advance
the time by a week or two. Set the Time Step to 5 minutes and animate. You can see the
Earth rotate, the Moon doesn’t orbit Earth very fast so the fact that we are viewing from the
Moon isn't obvious. Stop the animation.
Now set the Time Step to 1 day. Before starting the animation, try to predict what you
will see. Ready? Okay, animate. Is this what you expected? By using 1-day steps, Earth
always appears with the same side facing the Sun (except for some north-south variation
due to the tilted Earth). The changing view is because the Moon is going around Earth.
In about 30 steps (30 days, one month), the Moon makes a full orbit around Earth and
the view repeats. Notice how the Earth seems to grow and shrink in size. That’s because the
Moon’s orbit is an ellipse, the distance between the Earth and Moon varies (Earth appears
biggest when the distance is smallest). Stop the animation.
Planet Declinations
Select the File/Open Recent Settings/Startup menu yet again. Set the date to the
Summer Solstice (6/21). Select the Tools/Planet Report... menu, and then select Equatorial
Positions from the pop-up menu in the upper left of the Planet Report window (often hiding
behind the Time Panel).
Note the declination given for the Sun, it should be more than +23°. This is telling us
the Sun is directly above a location on Earth between 23° and 24° north latitude. The Sun is
shining more on the northern hemisphere because Earth is tilted towards the Sun on this
day. On an equinox, the declination of the Sun is 0°, it is directly above the equator. On
Winter Solstice, the declination of the Sun is -23.5°.
Before you close this window, notice the various declinations given for the other
planets. Mostly these declinations are a result of whether Earth is tilting towards or away
from the planets. 16. Why does Mercury usually have a high declination (positive) during summer and a
low declination (negative) during winter?
11
Mercury orbits close to the Sun so it always appears near the Sun in our sky. In the
summer, when Earth is tilted towards the Sun, both the Sun and Mercury appear higher in
our sky.
Phases of Venus
Open the Settings File called "Phases of Venus", read and close the text window, also
close the Planet Panel. If you are not seeing two windows, click the window-tiling icon
at the right end of the menu bar (the middle of those three icons)
. The view in the
Close-Up window is like what you might see through a telescope that magnifies 1300x while
the Wide Field view is more like what you see outside with the naked eye.
You may be disappointed that Venus appears so bland, without the photographic
quality you've seen for the Earth and Moon. Venus is completely and permanently
surrounded by thick layers of white clouds. The lit side of Venus appears in a telescope as
just a bright, white area. So, the computer display isn't so far off from what Venus really
looks like.
Animate and answer the following questions based on what you see.
17. Venus starts to appear smaller and smaller, what causes this?
Earth and Venus are moving apart. When Venus appears smallest is when Venus is
furthest away from Earth – when Earth and Venus are on opposite sides of the Sun.
Stop the animation when Venus is very near the Sun.
18. Which is closer to us (Earth) at this time, Venus or the Sun? You should be able to
figure this out without having to check any distance values.
The Sun is closer to us (Venus needs to be on the far side of the Sun so that we can be
seeing the same side that is being hit by sunlight).
Continue the animation, stop when Venus again appears very near the Sun, this will
be just after Venus starts its retrograde motion.
19. Explain why Venus appears so large and dark.
Venus is getting farther and farther away from the light of the sun Phases of Mercury
12
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Open the Settings File called "Mercury & Venus Phases". The geometry of the Earth,
Sun, and Mercury is basically the same as with Venus, hence the pattern of phases for
Mercury is just like for Venus. When Mercury is closest to Earth, it is in-between the Earth
and Sun and appears as a New or Crescent phase. When Mercury is on the far side of the
Sun away from Earth, it appears smaller and in the Full or Gibbous phase.
Start the animation. Mercury is the planet closer to the Sun following the brown orbital
path, Venus follows the white path, and Earth (along with our viewing location) is moving
along the blue path.
Both Mercury and Venus are orbiting faster than the Earth. When they are between the
Earth and Sun, they appear large and dark. When off to the sides they appear as crescents
and when on the far side of the Sun they appear small and fully lit. Note that planet sizes
have been grossly exaggerated in this demo.
20. Why are the stars moving across the screen? Why are they moving left to right?
Earth is moving, orbiting the Sun in the same direction that Mercury orbits the Sun.
The computer view is constantly turning to keep the Sun centered and this is why the stars
appear to move.
Q. Why does Venus appear to orbit so slowly?
A. We are viewing it from the moving Earth. Venus moves only a little faster than the
Earth so it only slowly catches up to and passes the Earth.
Phases of Mars
Open the Settings File called "Phases of Mars". The bottom-left window shows a close-
up of Mars as seen from Earth (you may have to push the window-tiling button again to see
both windows). The top window shows the corresponding positions of the planets in the
solar system at the same time. The planets in the top view have all been magnified to 1000x
their real size.
Start the animation. Like for Mercury and Venus, Mars appears smallest and fully lit
when on the far side of the Sun away from Earth. But Mars also appears fully lit when it
appears largest.
21. Explain why Mars appears fully lit when it looks largest while Mercury and Venus
appeared fully dark when largest.
Mercury and Venus orbit the Sun closer than the Earth; when closest (appearing
largest) they are between the Earth and Sun so that from Earth we see only the side away
from the Sun (dark side). Mars orbits outside Earth, when closest together we look at Mars
from the same direction the sunlight is coming from.
13
22. When does Mars appear half-lit from Earth?
Never.
Seasons of the Inner Planets
Open the Settings File called "Inner Planet Seasons". This is not
meant to be animated.
The Sun is, obviously, off to the right in each of the four windows. Each planet is being
shown when its northern hemisphere is tilted its maximum towards the Sun. You can see
that Mercury and Venus are hardly tilted at all while Earth and Mars have similar amounts
of tilt.
Open the Settings File "Mercury from Sun". The first thing you’ll likely notice is that
the Voyager program only shows surface features for half of Mercury’s surface. The first of
two spacecraft to approach Mercury was Mariner 10 from 1974 to 1975, it mapped only
about 45% of the planet’s surface (which is why the Voyager program shows just 45% of the
surface detail). The second was the MESSENGER spacecraft, which entered orbit around
Mercury on March 18, 2011; it mapped the entire surface, but that data is not part of the
Voyager program.
While Mercury's rotation axis may not be tilted, it can still be said to have seasons.
This is a view of Mercury from the Sun, animate and watch Mercury's weird dance. It gets
bigger and smaller because Mercury's orbit is elliptical, it moves significantly closer to and
further from the Sun during its 88-day orbit.
When Mercury is closer to the Sun, it moves along its orbit faster. In fact, at perihelion
(closest to the Sun) its spin rate (it rotates once every 59 days) and orbital motion match so
that Mercury temporarily keeps one side fixed towards the Sun.
If you lived on Mercury, the time from one sunrise to the next would be 176 days,
that's two Mercury-years! With the prolonged daytimes and being the closest planet to the
Sun, you'd expect it to have the highest temperatures but it actually ranks second (to
Venus). Because the nights on Mercury are so long, it can get very cold there. Mercury is the
coldest planet in the solar system!
Open the file "Venus from Sun" and animate.
Venus rotates very slowly (in 243 days) and rotates backwards (the other planets spin
in the same direction that they orbit the Sun but not Venus). Because of the slow spin, you
might expect Venus to have very different day and night temperatures. It doesn't.
Venus is surrounded by a blanket of thick clouds. These absorb sunlight and hold onto
it, like a greenhouse will trap heat on Earth. This causes the temperatures on Venus to be
incredibly hot, even hotter than Mercury. The U.S. and Russia have managed to land a few
spacecrafts on Venus, they sent back a few pictures and other data before they succumbed
to the hellish conditions.
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Seasons of Outer Planets
Open the Settings File called, you guessed it, "Outer Planet Seasons" (you should see
four small windows, if not click the window-tiling button again). All four of these "gas
giant", "outer", or "Jovian" planets have similar rotation rates. Jupiter and Saturn spin once
every 10 or 11 hours, Uranus and Neptune in 16 or 17 hours.
The four are shown on-screen with their correct relative sizes and all at maximum tilt
towards the Sun.
23. Which planet has the largest tilt?
Uranus
Uranus is very odd, it's the "planet tilted on its side". As Uranus goes around the Sun -
which takes 84 years - its rotation axis is carried so that it sometimes points towards the Sun
and sometimes away. If you lived on the North Pole of Uranus, you would have 42 years of
day followed by 42 years of night.
Even the rings of Saturn can be said to have seasons. Open the Settings File called
"Saturn Ring Seasons". This is a view of Saturn and its rings from Earth, like what one might
see through a powerful telescope. Start the animation.
24. Why does Saturn pulsate larger and smaller?
Because we are viewing from the Earth which is orbiting the Sun. That orbit takes us
repeatedly closer to and further from Saturn.
25. Why does Saturn slowly tilt up and down?
Saturn is always tilted the same direction and the same amount. But as it orbits the
Sun, it can change from tilted towards us to tilted away.
Twice during Saturn's 29.4-year orbit, we see it and its rings exactly from the side.
Then the extremely thin rings are edge-on and they disappear from our view!
Seasons Summary
Seasonal variations occur when parts of a planet receive extra or less sunlight for
extended periods of time. This can happen because of an elliptical orbit that moves the
planet closer or further from the Sun. It can also happen because of an unusually slow
rotation rate.
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But the most common source of seasons is a tilted rotation axis. This causes the
northern and southern hemispheres of the planet to get more or less sunshine depending on
where the planet is in its orbit. Seasons on Earth are almost entirely due to its 23.5° tilt.
The rotational tilts of the planets are:
Mercury
0.0°
Venus
2.7°
Earth
23.5°
Mars
25.2°
Jupiter
3.1°
Saturn
26.7°
Uranus
97.9°
Neptune
29.6°
Answers to Questions
1.
Answer omitted.
2.
Because only at noon on that day did Eratosthenes know exactly where the Sun was in
Syene’s sky (straight overhead, zenith) to compare with the Sun’s position in
Alexandria’s sky.
3.
Answer omitted.
4.
Answer omitted.
5.
Answer omitted.
6.
Answer omitted.
7.
Answer omitted.
8.
Higher, the local time in New York is 3:07 PM, it’s 12:07 PM in Bakersfield.
9.
The Sun! Or, more accurately, the place on the Earth for which the Sun is straight
overhead.
10.
Bakersfield by about 7 minutes.
11.
Because Las Vegas is substantially east of Bakersfield.
Earth rotates from west towards east. That’s why things rise in the east and set in
the west, it’s not that they are moving around the Earth but that Earth is whirling us
around. Las Vegas is east of Bakersfield; they are ‘ahead’ of us and see the Sun rise
before we do. Earth must rotate for an extra 15 minutes or so before we can first see the
Sun from Bakersfield after they’ve seen it in Vegas.
People on the east coast are about 3 hours ‘ahead’ of us. The Sun will rise and set
as seen from Bakersfield about 3 hours later – at least compared to cities at about the
same latitude (like Raleigh, North Carolina).
12. On the summer solstice, all places within the northern arctic circle get 24 hours of
daylight.
13.
Around June 21.
14. (d) never changes. That's not quite true. As we will learn in the next lab, the tilt does
change slowly in a cycle that takes 27,000 years. Polaris is only currently the North Star.
15.
Answer omitted.
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16. Mercury orbits close to the Sun so it always appears near the Sun in our sky. In the
summer, when Earth is tilted towards the Sun, both the Sun and Mercury appear higher
in our sky.
17. Earth and Venus are moving apart. When Venus appears smallest is when Venus is
furthest away from Earth – when Earth and Venus are on opposite sides of the Sun.
18. Assuming you stopped the animation when Venus appears fully lit in the Close-up
window, the Sun is closer to us (Venus needs to be on the far side of the Sun so that we
can be seeing the same side that is being hit by sunlight).
19.
Answer omitted.
20.
Earth is moving, orbiting the Sun in the same direction that Mercury orbits the Sun. The
computer view is constantly turning to keep the Sun centered and this is why the stars
appear to move.
21.
Mercury and Venus orbit the Sun closer than the Earth; when closest (appearing largest)
they are between the Earth and Sun so that from Earth we see only the side away from
the Sun (dark side). Mars orbits outside Earth, when closest together we look at Mars
from the same direction the sunlight is coming from.
22. Never.
23.
Answer omitted.
24. Because we are viewing from the Earth which is orbiting the Sun. That orbit takes us
repeatedly closer to and further from Saturn.
25.
Saturn is always tilted the same direction and the same amount. But as it orbits the Sun,
it can change from tilted towards us to tilted away.
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