Lab 5: Tracking the Night Sky

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Introduction

Observing the cosmos from Earth has its complications. Earth is a moving platform! It spins on its axis once a day, and travels hundreds of millions of miles around the Sun each year. In this lab we will track the apparent motions of the stars in different parts of the sky that result from the Earth’s rotation. The daily apparent motion of the stars is called diurnal motion and is shared by the Sun, Moon, planets, and other celestial objects. Along the way, we will learn to use the “RA/Dec” coordinate system that describes the locations of stars on the Celestial Sphere. We will also use our star wheels to predict when stars will rise and set, and when they are highest in the sky.

Part I: Apparent Motions of the Stars

Let’s take a close look at how our view of the skies changes over the course of the night as a result of the Earth’s rotation. To do this we are going to use Stellarium (https://stellarium-web.org/). Follow the directions below to set it up for the tools you’ll need for this particular lab.

1. Set the correct location. We are all going to use San Francisco as our location so that we can all see the same constellations. At the bottom of the webpage you should see a button that tells you where you’re observing from. It’ll say “near (location),” click that button. Once the map pops up, drag the location pin to San Francisco and click “> use this location” above the map. Also make sure that the toggle for ”Use Autolocation” is turned off.

https://stellarium-web.org/

2. Set the correct time. At the bottom right of the screen you should see another button with the time and date. Click that button. Once the window pops up you should see something that looks like the image below. Set up the date and time to match the image below. Set it for June 25th 2020 at 10:00pm (22:00 hours). Also be sure to “pause” time. You’ll see the pause button in the middle of the popup.

3. Turn on/off icon features. At the bottom, you’ll also see a bunch of symbols which will turn on and off certain features of the night sky. If you hover your cursor over the symbols, they should tell you what each one is. Turn on the “Constellations” symbol (leave off the constellations art symbol this time) and the “Azimuthal Grid” symbol. Turn off the “Atmosphere” symbol as shown below.

4. Turn on the Meridian. Click the three horizontal line icon in the top left of the screen and look for “View Settings”. Open the settings and check the box that says “Meridian Line”. Once done, you can close out of the settings menu. If you look around the sky, you should notice that the Meridian is a line in the sky (much like the Prime Meridian on Earth) that goes from due North on the horizon through your Zenith (90°altitude) to due South again on the horizon. It essentially divides the sky into an Eastern and Western halves.

Now that we have Stellarium setup so that we can use it’s useful tools, let’s learn how the sky moves from the perspective of Earth. Patterns that the stars make as the Earth turns will be different in different parts of the sky. Let’s focus on the northern and southern skies in particular.

Looking North

In Stellarium, turn your view so that you are looking North. You should see the Meridian line going straight down to the “N” on the horizon. You should also see the constellations Ursa Major and Ursa Minor. See if you can use what we learned during the last couple of labs to use the Big Dipper to point out Polaris. You should find Polaris nearly on top of the Meridian line.

1. Use the Azimuthal coordinate lines to determine the altitude of Polaris. If you look at the edge of the screen each line should have a marking in degrees (this is the altitude).

2. Does the altitude of Polaris match the location’s latitude that we set for ourselves in Stellarium?

Looking North, find the constellation Cassiopeia and click on the constellation name. You should see the art work and an information box about Cassiopeia pop up at the top left of the screen. Cassiopeia has an asterism associated with it, you may notice that it looks a little bit like a “W”, then it would make sense that its asterism is called The “W”.

3. What does the information box say about what (or who) the constellation was named after?

Now let’s close the information box and focus on the stars themselves. Take a look at the figure on the next page. Notice the degree scale on the left, and the letter “N” indicating the cardinal direction north along the horizon. Let’s sketch the constellation Cassiopeia as it appears at four different times over the course of the night.

1) Start by carefully placing Polaris on your sketch, making sure it is at the correct altitude. 2) Then sketch Cassiopeia and record the time. Again, pay attention to the altitude and its orientation.

3) Now let’s change the time by moving ahead 6 hours. You can do this by going to the time at the bottom right corner of the page. Change the hour from 22:00 to 04:00 . You can do this by using the up and down arrows or by using the slider.

4) Repeat steps one and two for this new time and two other times (10:00 and 16:00 ) which will represent the course of a night.

5) Don’t forget to record the times in the space provided!

Use your sketch to answer the following questions: 4. What pattern on the sky does each star trace out in 24 hours?

5. In what direction do the stars move? (clockwise or counter-clockwise?)

6. Are there any stars that appear not to move at all?

Looking South

Now orient the Stellarium view to the south. You should again see the Meridian but this time going straight down to the “S” on the horizon. We’ll again sketch a constellation at multiple times to see how things change over the course of the night.

To start, turn the time back to 22:00. Then, while looking south find the constel- lation Sagittarius. Sagittarius is one of the Zodiac constellations, but we will learn more about them in a later lab. However, Sagittarius also has an asterism associated with it, called The Teapot. If you look closely you may see a shape that resembles a teapot.

Once you locate Sagittarius, draw it (keeping note of the altitude and orientation) and write down the time. Do this for two more times (01:00 and 04:00), each 3 hours apart.

Use your sketch to answer the following questions: 7. What is the overall pattern of motion of stars in the southern part of the sky?

8. Are there any stars that appear not to move at all?

9. If a star rises in the southeast, where does it set?

10. If a star rises due east, where does it set? (hint: try to use Stellarium to find a star that rises due East and follow it for a whole night until it sets.)

11. What is the cardinal direction (N, E, S or W) of a star when it is highest in the sky (on the Meridian)?

Make a prediction: Starting at its current position, how long will it take for a constel- lation to get back to its original location?

Try it!: Find a constellation, turn time at least 24 hours and watch where it goes. How good was your prediction?

Part II: Tracking the Sky with a Star Wheel

Now let’s move to using your Star Wheel. We saw in Lab 2 that a Star Wheel is very useful for identifying constellations. That’s really only the beginning of what a Star Wheel can do. Here we’ll learn how it can be used to predict when a star will be highest in the sky, when it will rise and set, and in what direction you should look for it at any given time.

Meridian Crossing Times

The best time to observe a star (or other astronomical object) is when it’s as high in the sky as possible. That’s not only because it’s more likely that it won’t be hidden behind a tree or building, but also because the star will be passing through the least amount of atmosphere. The more atmosphere it passes through, the dimmer the star will tend to look, and the more distorted its image will be. To the unaided eye, this distortion manifests itself as “twinkling;” stars that are low in the sky tend to twinkle more than those higher up. When looking through a telescope, things that are low in the sky look quite blurry, which makes it hard to see all the interesting fine details!

1. Recall the observations made using Stellarium in Part I of this lab. Consider stars that rise in the east or southeast. What cardinal direction are stars in when they are highest in the sky?

To determine when a star is as high as it can be, you can look for when it crosses the meridian. You can mark this line on your star wheel (both sides!). It runs from the point due north on the horizon, through the zenith, to the point due south. See the picture below so that you can also draw this on your Star Wheel.

Figure 1: Front of Star Wheel Figure 2: Back of Star Wheel

Note, that the time that a star will cross the meridian depends on the date, for reasons we’ll learn in a future lab. Try these problems (you can assume standard time, i.e. Star Wheel time, throughout). Let’s walk through this first one together:

On Sept. 20, at what time will Altair, the southernmost star in the summer triangle, be due south?

We know to be due South a star needs to be ON the Meridian. Find Altair on your Star Wheel and move the wheel until Altair is on the Meridian line that you drew. Now, without moving the wheel anymore, find Sept. 20 along the edge of the Star Wheel. Once you find that, notice the clock times around the blue part of the edge of the Star Wheel. There should be a time pointing to (or near) Sept. 20. In this case Sept. 20 is near 8pm, or to be more precise, about 7:50pm. Now we know that if we want to observe Altair when it is highest in the sky on Sept., 20 we need to look for it around 8pm.

The following questions are variants of the one above. In any of these questions you need 3 pieces of information, an object, a date and a time. You are given two and you need to find the third. In the above example you are given a date and an object, you needed to find the time.

2. If you see the globular cluster M22 in Sagittarius due south at midnight, what date is it? (hint: you are given the object and the time, from that put M22 on the Meridian, look for midnight and your answer is the closest date.)

3. On what date can you see Sirius, the brightest star in the sky, due south at 9pm?

4. On Nov. 1, what time would be best to take a picture of M31, the Andromeda galaxy?

Rise and Set Times

Before using the Star Wheel to determine when a star will rise or set, we need to know which way to turn the wheel. The answer differs depending on which side of the star wheel you are using!

5. Look at the “front” side of your star wheel (the side with the oval-shaped opening). Which way to do you need to rotate the wheel so the stars rise in the east and set in the west? (clockwise or counterclockwise?)

6. Now turn over your star wheel (to see the southern skies). Which way do you need to rotate the wheel now to make the stars rise in the east and set in the west?

Where in the Sky

The cardinal direction of a star or constellation can be a little trickier to figure out from your star wheel if it’s not near the horizon. Here’s how you do it. Remember that cardinal direction is determined by dropping a line from the star straight down to the horizon and asking what direction that is. To simulate dropping this line straight down, imagine a line on your star wheel running from the zenith down to the star or constellation you are interested in. Then extend that line straight down until it touches the horizon. The point where it touches the horizon is the cardinal direction of the star. This method works well for stars in the north, east, west, and in between (when you’ll use the front of your star wheel). For stars in or near the south you’ll use the other side of your star wheel. In those cases, drop a line from the star down to the horizon, making sure the line is perpendicular to the horizon, and again read the cardinal direction off the horizon.

Now use your star wheel to answer the following questions.

7. On October 1st, you decide you want to look for Orion. To determine what range of times it will be visible that night, rotate the wheel until Orion begins to appear above the horizon.

a) On Oct. 1, at what time will Orion’s belt rise?

b) What direction would you need to look to see it rising?

c) Will Orion still be up just before the Sun rises, at 5 am?

d) If so, what direction should you look for it?

8. On November 1st, you decide to look for the summer triangle asterism, which has been visible all summer and into the fall, but by winter will have disappeared from the evening sky.

a) On Nov. 1, in what direction should you look for the summer triangle at 7pm standard time?

b) Which of the three bright stars in the summer triangle will set first and it what time and direction will it set?

c) Which of the three bright stars in the summer triangle will set last and it what time and direction will it set?

9. On March 1st, you decide to use binoculars to look at two Messier objects: M31, the Andromeda Galaxy, and M45, the Pleiades star cluster.

a) At 7pm, what direction should you look for Andromeda?

b) At what time will Andromeda set that night?

c) At 10pm, what direction should you look for the Pleiades?

d) What time will the Pleiades set that night?

10. On April 1, you decide to test your eyesight by looking for Alcor, a star 0.2 degrees away from Mizar, the middle star in the Big Dipper’s handle. Also known as the “horse and rider,” Mizar and Alcor have been a test of eyesight since ancient times.

a) At 7pm that night, in what direction should you look for Mizar and Alcor?

11. On June 21st you are on a camping trip to celebrate the summer solstice, hoping to see Scorpius and Sagittarius for the first time. These are particularly interesting con- stellations: Scorpius dates all the way back to Babylonian times; its bright reddish star Antares (“rival of Mars”) is a supergiant with a diameter more than 800 times that of the Sun. Sagittarius marks the direction to the center of the Milky Way: the massive black hole at the center of the Galaxy lies about 27,000 lightyears in a direction just off the tip of the spout of the “teapot” asterism.

a) What time will Antares in Scorpius rise that night?

b) In what direction will you need to look to see it rising?

c) What time will the star at the tip of the “teapot” asterism in Sagittarius set?

d) In what direction will the teapot set (and along with it, the Galactic center)?