Constellations and Asterisms
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Eastern Michigan University *
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105
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Astronomy
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Apr 3, 2024
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Constellations & Asterisms
On a clear night, far away from city lights, the number of stars you can see in the sky seems astonishing. They appear to be scattered in random fashion, but human nature soon places the brighter stars into easily recognizable patterns: lines, squares, circles. Throughout history, sky observers have joined the brighter stars into asterisms, patterns of stars
which represented animals, objects, or heroes from their various cultural mythologies. In the absence of movies or television, a storyteller could simply point to the sky and illustrate any story being told using any familiar pattern of stars they imagined.
These patterns would eventually form the 88 constellations of our modern skies, though most people have a great deal of trouble seeing a “horse” or a “dog” or a “hunter” in the star patterns. Remember that they are only fictional representations
used as a reference for navigating the sky! The scientific definition of a constellation is “any one of 88 areas of the sky as designated by the International Astronomical Union”, akin to having state boundaries on a map. Ancient peoples recognized long ago that the Sun appeared to move along a certain path, now called the ecliptic
, through a select band of stars in the sky throughout the year
. This illusion is the result of the Earth’s orbital motion around the Sun. The constellations found along the ecliptic belong to a privileged set called the Zodiac Constellations
. Another special set of constellations that never sets below the horizon is called Circumpolar Constellations
. You can download Stellarium for free from https://stellarium.org
on your own computer, or use the online software available at https://stellarium-web.org
. The Stellarium program serves as the basis for the EMU Planetarium projector system. Complete the following worksheet based on what you observe. Through this activity you will virtually observe constellations, asterisms, and important objects and locations in the sky. This information will be helpful for completing the non-lunar observations this semester if you are unable to attend observing
sessions at Sherzer Observatory.
Worksheets
Answer the following questions using the Stellarium software or https://stellarium-web.org
.
1. As the Earth orbits the Sun once per year, four different season groups of constellations present themselves in our southern sky. Write down the names of 3 noteworthy constellations located in the southern sky for each of the four times and dates listed below. Winter (1/21/2024 22:00:00):
Spring (4/21/2024 22:00:00):
Summer (7/21/2024 22:00:00):
Autumn (10/21/2024 22:00:00):
Next, while facing north, identify 3
North Circumpolar constellations, located closest to the NCP – North Celestial Pole. (These are constellations that never completely set below our horizon.) Polaris is located very close to the NCP, which is why it is known as the Northern Star
.
North Circumpolar (1/8/2024 22:00:00):
2. The path the Sun appears to take through the background stars over one Earth orbit is called the ecliptic
. If you are using the Web version of Stellarium, you can turn on the ecliptic by clicking the “View Settings” option (with a gear next to it, in the menu on the left of the screen) and checking the option for “Ecliptic Line.”
Turn on the ‘constellations’ and turn off the ‘landscape’ in Stellarium, then search for and lock onto the sun (Stellarium should automatically lock onto the sun if you search for it). Advance time by
single-day increments (and/or single-month increments) and observe the sun as it moves through the 13 zodiac constellations. To see the constellation boundaries, click on each of the constellation names. Write down the names of the 13 zodiac constellations in the correct order the sun passes through them, starting from 1/8/2024 22:00:00.
3. Reset the date to 1/7/2024 22:00:00, and find the bright planet, Jupiter. What constellation is Jupiter currently in this year?
Jupiter generally moves eastward along the ecliptic (very slowly). What constellation might we expect to find this giant planet exactly one year later? Use the simulation to determine the correct answer. 4.
The Vernal Equinox (VE) is a point in the sky where the ecliptic intersects the celestial equator (a projection of the Earth’s equator into the sky). The sun is located at the VE on 3/20/2024 at 11:32 a.m. To more easily see the location of the VE, set this date and time in the simulation, and search for the Sun. Turn on the equatorial grid by clicking on the “Equatorial Grid” button, and notice the gridline that crosses through the Sun on this date; this line is the celestial equator. When the sun is centered on the VE, it signals the start of spring in the northern hemisphere. That coordinate point is still called “The First Point of Aries” even though it has been slowly drifting westward along the ecliptic for centuries due
to precession. What constellation contains the VE in modern times?
5. The pseudoscience of astrology
suggests that the Sun’s position among the stars on the day you were born provides insight into your personality and allows people to predict events in your life based on the apparent positions and motions of celestial objects. Most people know the “sign” (constellation) astrology attributes to their date of birth and can read daily, weekly, or monthly horoscopes based on that information. What is your sun sign according to astrology? (If you aren’t sure, google ‘Sidereal and
Tropical Astrology’ and look up the tropical dates in the Wikipedia link!)
Due to precession (and human design), the actual position of the Sun has very likely shifted one constellation west of the sign you are given. Set the date to that of your birth (month, day and year). What constellation was the Sun in on the day of your birth? Are they the same?
6. Set the date to 1/8/2024 22:00:00. In which constellation will the Moon appear today?
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Knowing that the Moon, Sun, and planets travel eastward along the ecliptic over varying times, what constellation might you expect the Moon to visit a few days later along the ecliptic?
Use the simulation
to determine the correct answer (advance time by single-day increments). 7. The North Star, Polaris,
an important star in navigation, is readily found in the night sky by following a line drawn from the two leading stars of the Big Dipper asterism, named Merak and Dubhe (also called “The Pointers”). (Remember from Question 1 that Polaris is located very close to the North Celestial Pole
(the NCP),
located above the North point of our horizon.) In which constellation do we find Polaris? 8. When you look up at the night sky, in real life, can you actually see the constellations (the dashed boundaries that house those areas of sky)? Can you see the dashed lines connecting the stars of the asterisms?
9. Click on a few of the constellation names and look at the illustrations that appear. In your opinion, do
the asterisms that represent each illustration accurately reflect the illustration they are supposed to represent?