Observing the Night Sky: Angular Measurements & Constellations

AST 1120: Stellar Astronomy Name: Lab 10 (40 points) Observing the Night Sky Learning Goals  Measure the angular sizes and positions of objects on the sky  Describe how stars will move throughout the night, as seen from Colorado  Identify prominent stars and constellations visible in the northern hemisphere Part 1 – Angular Measurements To measure angles on the sky, we use degrees, arcminutes and arcseconds. 1 degree = 60 arcminutes (60’) 1 arcminute = 60 arcseconds (60’’) Figure 1 - example of how 1 degree of a circle can be broken up into 60 arcminutes. 1 arcminute can be broken up into 60 arcseconds Examples: Andromeda galaxy is roughly 2 degrees on the sky. To convert this into arcseconds (’’) 2 °× 60 ' 1 ° × 60 ' ' 1 ' = 7200 ' ' 1. The full moon measures about ½ a degree on the sky. How many arcseconds is this?

2. The angular size of Mars at a recent opposition was 20 arcseconds across. How many Mars would fit across the size of the full moon? Look at previous question. Figure 2 - The Horizon System measures coordinates on the sky with altitude (measured up from the horizon) and azimuth (measured along the horizon starting at north) Altitude measures an object’s position above the horizon, from 0 ° at the horizon to 90 ° at zenith. Azimuth measures an object’s position along the horizon, starting at North ( 0 ° ) going to 90 ° at East, 180 ° at South, 270 ° at West, and back around to North. Horizon System Interactive has an animated version of this system. Degrees are quite large. A degree can be divided into 60 arcminutes. An arcminute can be divided into 60 arcseconds. Many of these concepts were covered in Chapter 2. 3. What is the Altitude of zenith?________________________ 4. What is the Azimuth of South? _____________________ 5. Approximately what is the altitude of the example star shown in figure 2: _______ 6. Approximately what is the azimuth of the example star shown in figure 2:_______ 2

Figure 3 - Figure showing the position of the North Celestial Pole on the sky for observers at a) north pole b) equator and c) intermediate latitude 7. Standing at the north pole, what is the altitude of the north celestial pole? ______ 8. Standing at the equator, what is the altitude of the north celestial pole? _______ 9. What is the approximate latitude of your current location? ____________ (google maps knows) 10.What is the altitude of the north celestial pole at your location? _____________ Part 2: The Circumpolar Zone Use this online planisphere to answer the following questions. You can zoom in to make the planisphere larger (ctrl + on a PC). Please ignore the date at the bottom of the planisphere. The planisphere shows the sky at many different dates and times. 11.Click and drag on the top of the planisphere until the beginning of May lines up with 9 PM. Then drag early May from 9 PM to 5 AM and list four constellations on the planisphere that will set over the course of a night in May. 12.Click and drag on the planisphere to move early May from 9 PM to 5 AM and list four constellations on the planisphere that will rise over the course of a night in May. 3

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13.Click and drag the planisphere all the way around several times. List four constellations on the planisphere that are visible all the time. 14.Explain why some constellations are circumpolar for observers in Colorado. Part 3: May Night Sky for the Northern Hemisphere Download the Sky Map for this month’s Night Sky (northern hemisphere). Answer the following questions using the May sky map for the Northern hemisphere. Remember that asterisms are not the same thing as constellations. 15.What constellation is closest to zenith on this sky map? 16.What named star is closest to zenith on this sky map? 17.What constellation contains the north pole star, Polaris? 18.Why is Polaris not located at the zenith as seen from Colorado? 19.Which constellation did you choose for your presentation this semester? Is this constellation visible tonight on this sky map? 4

20.Which Messier object did you choose for the Messier discussion this semester? Is this Messier object visible tonight on this sky map? 21.List five of the brightest stars visible on this sky map (larger size indicates greater brightness): 22.List five of the constellations that the Milky Way passes through this month. 23.Can you find any constellations or asterisms yourself in the night sky? If so, which ones? (Not just in May, but any time of year) 24.Explain what the ecliptic represents on the sky. 25.List any planets that are visible this month. Use the Calendar on the left side of the Sky Map. 26.Fill in the table below using the calendar on the left side of the Sky Map. After you have angular size in arcminutes, convert to angular size in arcseconds, 5

remembering that 1 arcminute = 60 arcseconds. Use only the 1 st Perigee/1 st Apogee listed, if there are more than one. For the Moon at Perigee : For the Moon at Apogee : Angular size (arcmin): Angular size (arcmin): Angular size (arcsec): Angular size (arcsec): Distance (km): Distance (km): 27.For the Moon at Perigee (d is distance), determine the linear size D of the Moon (in kilometers) using the equation below. Plug in angular size in arcseconds. Show your work. D = ( α ×d ) 206265 28.For the Moon at Apogee (d is distance), determine the linear size D of the Moon (in kilometers) using the equation below. Plug in angular size in arcseconds. Show your work. D = ( α ×d ) 206265 29.Average together your two values for the linear size of the Moon. 30.The true value for the linear size of the Moon is 3474 km. Determine the percent error in your calculation for the moon. ¿ 6

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Image from APOD 31. Why does the angular size of a full moon vary from month to month? 32.The distance to the Andromeda Galaxy (M31) is 2.537 × 10 6 light-years and its angular size on the sky is 10,800 arcseconds. Use these numbers to determine the diameter D of Andromeda. Your answer will be in light-years. Show your work. D = ( α ×d ) 206265 33.The accepted value for the diameter of Andromeda is 220,000 light-years. Determine the percent error in your calculated diameter for Andromeda. Show your work. | ( ( D calculated − D accepted ) D accepted ) | × 100 = ¿ 7

34.The accepted distance to the Andromeda Galaxy has quite a bit of uncertainty. The distance can be expressed as ( 2.54 ± 0.11 ) × 10 6 light-years. This is an uncertainty of 110,000 light years! Why is there so much uncertainty in measuring the distance to objects outside of our galaxy? Image from APOD 35.Explain why we are not able to see the Andromeda Galaxy this clearly from Colorado. 8

AST1120-lab10

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