# Motions in the Night Sky

Motions in the Night Sky

ASTR 1010L

The goals of this lab are to build on what you know about the night sky by learning how the objects in the sky move.

Get out the planisphere you constructed in the Introduction to the Night Sky and set it to see the sky tonight at 9 p.m.

Review your answers to Questions 9 through 13 of Introduction to the Night Sky and make sure you are still happy with them.

Let’s go back to the two constellations that you chose that are nearest the eastern horizon and watch what happens to them as the night progresses.

Which two constellations did you choose (Question 11 of the Introduction to the Night Sky)?

Orion and Monoceros constellations

Move today’s date so that it is aligned with 10 p.m. What happens to the position of your constellations?

The Orion constellation has moved slightly near the northern part of the hemisphere along the Orion belt. The Monoceros constellation has moved slightly towards the Southern hemisphere near the Canis Major constellation

Move the date so that it is aligned with 11 p.m., observe, then move the date so that it is aligned with 12 a.m., observe, then move the date so that it is aligned with 1 a.m. Describe what happens to your two constellations over four hours (from 9 p.m. to 1 a.m.).

The Orion constellation changes position from the Eastern hemisphere at 9 p.m, by 10 p.m, the constellation is close to the northern hemisphere. By 1.am, the Orion constellation is headed towards the North eastern hemisphere along the Orion belt. On the other hand, the Monoceros constellation moves slightly towards the Southern hemisphere near the Canis Major constellation at 9 p.m. At 10 p.m. the constellation is moving towards the South Western hemisphere. By 1.am. the constellation has moved towards the Western hemisphere.

Which of the constellations is closer to the zenith?

Ursa Major and Canis.

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Where is the other constellation?

While Ursa is slightly below zenith, Canis is slightly above zenith

Are the two constellations that were near the western horizon at 9 p.m. (Question 12 of the Introduction to the Night Sky) still visible?

They are no longer visible to the naked eye.

What happened to them?

They have moved quite far from the to the hemispheres further from the Earth.

Look again at the two constellations that started off (at 9 p.m.) on the eastern horizon. Where in the sky are they just before the sun rises at 6 a.m.?

Both the constellations have moved towards the western hemisphere. While the Orion constellation is in the North Western hemisphere by this time, the Monoceros constellation is moving towards the South Western hemisphere.

At 6 a.m., can you see either of the constellations that were visible on your western horizon at 9 p.m.? If so, where are they?

The constellations that were there at 9.pm. are now moving either to the northern or southern hemisphere by 6.am.

Take your planisphere back to 9 p.m. tonight. Now move the time from 9, to 10, then 11, 12, and 1 a.m., watching the constellations that were near your northern horizon at 9 p.m. (Question 13 of the Introduction to the Night Sky).

Describe their motion in the sky.

Constellations that were near the northern horizon are either moving towards the eastern or western hemisphere of the sky. By 10 p.m the constellations are either in in north eastern or not western hemispheres of the sky. Between 12 am. And 1.am. the constellations are headed towards either the eastern or western hemispheres.

Are they ever going to set in the west?

Yes, before 6.am, they would have settled at the west.

Slowly spin the star wheel all the way around so that tomorrow’s date is lined up with 9 p.m. What happens to these two constellations?

The two constellations return back to the same position they were at 9.pm the previous day.

Find Cassiopeia, if it wasn’t one of the two northern constellations you were watching. Sketch Cassiopeia below, paying attention to the orientation of the “M” that is shown on your planisphere when observing at 9 p.m.

Move your planisphere to 1 a.m., then sketch Cassiopeia again, again paying attention to the orientation of the “M” that is shown on your planisphere.

Move your planisphere to 5 a.m., then sketch Cassiopeia again, again paying attention to the orientation of the “M” that is shown on your planisphere.

Imagine you could observe Cassiopeia 4 more hours later, at 9 a.m. (It’s still there, but the daytime sky is too bright to see any stars except the Sun!) To do this, estimate getting today’s date and moving it around to where 9 a.m. would be on the planisphere. Sketch Cassiopeia again, again paying attention to the orientation of the “M” that is shown on your planisphere.

What do you notice about the orientation of your “M” compared with how it looked at 9 p.m.?

The orientation of “M” is less clear at noon compared to how it looked at 9 p.m.

Both the east-to-west motion of some of the stars as well as the “circling around” of the stars in the north is due to the Earth’s constant rotation. In order to visualize this, we’re going to utilize the online simulations you downloaded for the Introduction to the Night Sky lab. Open the NAAP Labs from UNL Astronomy (the downloads page is linked again in iCollege if you need it).

Choose “3. The Rotating Sky”, then open the Rotating Sky Explorer.

On the left is the celestial sphere view of the sky showing where you are standing on the Earth. Move the dot on the map in the lower left corner so that you are approximately in Atlanta (lat 34° N, long 84° W). You should see the celestial poles and celestial equator marked on the celestial sphere. There are also two circles shown that are perpendicular to the celestial equator; you may ignore these.

Add the Big Dipper, Orion, and the Southern Cross to your celestial sphere by clicking “star patterns…” in the Star Controls box in the lower right. Click “start animation”.

What happens to the celestial sphere (look in the “celestial sphere view” on the left) as the Earth rotates?

As the earth rotates, the celestial sphere rotates on the opposite direction of the earth’s rotation.

Why do you think this is?

It is because while the earth rotates round the sun from West to East, the celestial sphere rotates from East to West

Before proceeding with this animation, we need to understand the “horizon diagram view”. Go back to the NAAP labs main page and click on “Two Systems – Celestial, Horizon”. You are again seeing the celestial sphere view of the sky. You can’t control your location on the globe, but the

simulation places you at approximately the correct latitude. For what the simulation is trying to help you understand, though, your location doesn’t matter.

Click “switch”. What happens? How is the horizon view different from the celestial view? (You may want to switch back and forth a few times and watch.)

It is difficult to observe the whole celestial sphere from the Earth, as the horizon limits our view of it.

Go back to the first simulation (Rotating Sky). While the animation is going, watch Orion in the “horizon view”. Describe its motion.

I can see that the stars at the Orion constellation are moving from left (east) to right (west). Besides, at the horizon view, Orion is near the western horizon.

Which of the constellations that you named on your planisphere behave like Orion behaves in the simulation?

Lyra Constellations

Now watch the Big Dipper. What is similar and what is different about its motion as compared to Orion?

The Big Dipper moves around the Earth’s geographic North Pole. Unlike Orio which moves from east to west, the Big Dipper moves from west to east.

Which of the constellations that you named on your planisphere behave like the Big Dipper behaves in the simulation?

Ursa Major and Canis.

Finally, watch the Southern Cross. Compare and contrast its motion to the Big Dipper.

It moves like a big hour hand. It circles around the south celestial pole in a clockwise direction throughout the night.

Can you see the Southern Cross from Atlanta? Explain.

All right, back to our planispheres. As you spin the wheel over 24 hours, there is one star that hardly moves. Which star does it look like to you? Sketch the constellation containing the star, and circle which star you think it is.

Let’s go ahead and check your answer. The star that doesn’t move very much is at the end of the Little Dipper, labelled by its constellation name (Ursa Minor) on the planisphere. We call this star Polaris because it’s very close to the North Pole on the celestial sphere. This star is right under the S in Ursa Major on this planisphere.

Notice that this star is not very bright! The brightest stars in the sky are marked with s on the planisphere, while fainter ones are just dots.

You can find this star most easily by finding the Big Dipper in the sky, then using the two stars farthest away from the dipper handle to point to Polaris (also called the North Star). Sketch this below.

Polaris will not change position over the course of a night regardless of where a person is, but where Polaris appears in a person’s sky depends on where on Earth that person is located.

914400422488If you are on the North Pole (90° N latitude), Polaris will be directly over your head (at your zenith!) 90° above the horizon. If you are anywhere along the equator (0° latitude), Polaris will be 0° above the horizon – in other words, not really visible.

Look at the diagram below.

If you are somewhere near halfway between the equator and the North Pole – like we are in North America – where do you see Polaris (the North Star) in the nighttime sky? Add your position on the globe in the sketch above, and then draw a dashed line between you and Polaris (the North Star). Is Polaris over your head? On your horizon? Somewhere in between?

The Polaris is over my head on the since it is o n the north pole.

Does this agree with what your planisphere tells you about where Polaris is?

Yes it does since when the earth is rotating towards the sun in a clockwise motion, the Polaris remains at the north pole which is normally above at the horizon.

Stars and constellations that are close to Polaris – like Cassiopeia – simply circle around all day and all night without ever setting. These stars and constellations are called circumpolar. Which stars are circumpolar depend on where on the Earth you are located – the closer to the North Pole you are, the more stars and constellations that will be circumpolar.

What constellations are circumpolar in the latitudes for which the planisphere was designed (about 25° to 40° N)? (Watch the sky over 24 hours – which constellations never set?)

Ursa Major and Cassiopeia are circumpolar in the latitudes 25° to 40° N. The Big Dipper constellations never set.

How would you describe how the position of each of these constellations changes over the course of 24 hours?

After 24 hours, the Ursa Major and Cassiopeia are set and are at western hemisphere. However, the Big Dipper constellation continues to move, it does not set.

How would you describe how the orientation of each of these constellations changes over the course of 24 hours?

After 24 hours, the Ursa Major and Cassiopeia are set and are oriented same hemisphere as they were 24 hours earlier. However, the Big Dipper constellation continues to move, it does not set.

The circles the stars make around the pole is beautifully demonstrated in photography where the shutter is left open for a long period of time while the camera is pointed to the north.

Open “Circumstellar Star Photography” and look at the photographs. The top photo is a little distorted due to the wide angle lens being used, but the one halfway down the page is not distorted (the one captioned “The stars revolve around the North Star, which serves as the center of the great celestial clock.”). Roughly sketch something that looks like the photograph in the space below.

This article contains good information and animations – it’s a great resource to skim through to help you understand what you’re seeing with the planisphere!

Look at the diagrams in the “Circumpolar Stars in the Northern Hemisphere” image. Hopefully this will help you visualize why some stars rise and set and some never do – it depends on how close they are to the North Celestial Pole (or South Celestial pole, if you were observing from the Southern Hemisphere) and the latitude from which you are observing.

Take a few minutes to do your best to verbally summarize why some stars are circumpolar and why some stars rise and set, and what stars have the paths they do through the sky.

Circumpolar stars neither rise nor set, but stay up at all hours of the day, every day of the year. Even when you can’t see them when the sun is out and it is daytime.

Remember that how high in the sky the North Star (Polaris) is tells you your latitude. Where would you be if it were directly overhead?

In the north pole.

And if you were at the North Pole, all the stars and constellations you see would be circumpolar: you can only see down to the celestial equator, and nothing would ever rise or set because the sky would make a circle around the North Star which is directly overhead. The animated diagram “Circumpolar Stars at the North Pole” linked in iCollege shows this.

What about if you were on the equator of the Earth? Where in the sky would the North Star/North Celestial Pole be?

The North Star will be in the north direction.

What about the South Celestial Pole?

It will be overhead on the south pole.

So how many constellations would be circumpolar to you?

Two constelations

Check yourself by looking at the “Stars from the Equator” diagram on iCollege.

If you were in the southern hemisphere, your view of the sky would be different – you would see the stars that are above you “down there”.

Print out the “Southern Star Wheel”, if you did not do that when you printed out the planisphere.

Compare the two star wheels when they are NOT in the planisphere. What constellations are on both star wheels? (Note that the shapes of the asterisms may not look the same – this is because of the distortion of any two-dimensional map that represents three-dimensional space – it’s just like Greenland is distorted and looks huge on some map projections!)

At the “Southern Star Wheel” there are Ursa Major and Polaris. At the “Circumpolar Stars in the Northern Hemisphere” there are Big Dipper and Cassiopeia constellations.

You may also note that the order of months is opposite. If you had been given the whole planisphere for the southern hemisphere, you would see that the northern and southern horizon labels were reversed, so the eastern and western horizons would also be reversed. Stars still rise in the east and set in the west, but now they appear to rotate around the South Pole instead of the North Pole!

For our little project, though, you can just put the Southern star wheel into your planisphere. Notice that any constellations that are recognizable to you will be near the bottom of the visible window rather than near the top.

Rotate the star wheel around. Find the point that doesn’t appear to move in the sky as the stars rotate around.

Is there a star at this position?

Hopefully you noticed that there isn’t… There is no “southern Polaris” or “South Star”! Instead, to find true south, people in the Southern Hemisphere find what we in the Northern Hemisphere call the Southern Cross (we have our own “Northern Cross” that isn’t visible to them). They call it simply “Crux”. Crux is a small constellation. Find it.

Look for the two stars in Crux that are near the Cs in “CRUX” and “Acrux”, then draw an imaginary line from the one near the C in “CRUX” to the one near the C in “Acrux”, then keep going to the open space near Octans. The South Pole is along that line roughly above the C in “OCTANS”.

Either mark this spot on your planisphere or just watch it carefully as you spin the wheel around in time – do you see that it doesn’t move?

Yes, it does not move as I spin the wheel.

Another star of interest I’d like you to find is Alpha Centauri – this star system (containing three stars, including Proxima Centauri) contains the stars that are closer to our Sun than any others.

5923915-6731000Find the constellation of Centaurus. Alpha Centauri is one of the feet of the centaur – the one circled in the diagram to the right.

Is this a star you can see from the continental United States?

Yes, it is visible.

Why or why not?

It is visible because the star is in the southern hemisphere.

Ok, now back to the stars that we can see.

As you’ve seen, the stars that are visible to you change over the course of the night due to the Earth’s motion as it rotates every 24 hours. However, the stars that are visible to you also change over the course of a year as the Earth revolves around the Sun.

Put the Northern Hemisphere star wheel back in your planisphere.

Is there anytime during the night tonight (9 p.m. – 6 a.m.) that you would be able to see Orion?

It can be seen between 9 p.m. to 10.pm.

During what months would you be able to see Orion at 9 p.m.?

November to February.

Let’s look at why this is. The diagram below shows the Earth orbiting the Sun, then the celestial sphere of stars surrounding us. Of course in real life, the stars would be MUCH farther away – Alpha Centauri, the largest star in the closest star system, is a star a lot like our Sun. If you imagine putting it in the model you constructed for the first lab, it would be over 250,000 times farther away from us than the Sun is from the Earth!!!

265905742373837040613406174262187459362213655675622447846897799733229060304993Stars of Ophiuchus

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319341519939000 Earth in mid-June

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3846560-133903229060-13390Stars of Orion

The stars would also be in a big three-dimensional sphere instead of just a circle, but since the Earth orbits the Sun in a plane, just taking a slice of the celestial sphere near the ecliptic works just fine for our visualization. The stars are also varying distances from the Sun and Earth. However, just like with the size of the celestial sphere shown on the diagram, this doesn’t matter to understand about why you see some stars at different times of year.

The location of the Earth is shown in mid-June. Put another dot on Earth’s orbit where Earth will be in 6 months, in mid-December.

How do you know where to put your dot?

At the center of the circle.

According to the diagram on the previous page, can you see the stars that make up Orion when it is June?

The stars in Orion can only been seen between November and February for people in the northern hemisphere. Therefore, I will not be able to see the stars that make up Orion in June.

Remember that the Earth is rotating every 24 hours at the same time that it is revolving around the Sun. When you are on the part of the Earth facing the Sun, it’s daytime, and when you are on the part of the Earth away from the Sun, it’s nighttime.

If the Sun didn’t cause our daytime sky to be bright (blame the atmosphere for that – on the Moon you can see stars even when it’s daytime, as long as you don’t look in the direction of the Sun itself…), then when would the stars of Orion be right in “front” of you, according to the diagram? (This is equivalent to being close to overhead in the sky.) Noon, right?

Set your planisphere for today at noon. (Make today’s date be opposite 12 a.m.; “June” should be right where the planisphere says “Southern Horizon”.) Where is Orion?

Orion would be in the north pole. As the earth rotates round the sun, the position of Orion changes.

Based on the diagram on the previous page, at what time of night in mid-June will the stars of Ophiuchus be overhead? Hint: it’s when the Sun is completely behind you, roughly halfway between sunset and sunrise…

Set your planisphere for this time. Were you right? Explain how you know.

Yes, I was right, the stars of Ophiuchus in the planisphere were overhead at midnight .

Use your planisphere to determine what constellation will be overhead (or at least high in the sky) at midnight in mid-September. What is it?

Microscopium constellation

Use your planisphere to determine what constellation will be overhead (or at least high in the sky) at midnight in mid-March. What is it?

Canis Minor constellation

NOTE: Students often struggle with the last few and the next few questions. If you’re confused or unsure, please be in touch with Dr. Skelton by text or email!

In order to label the locations of these stars, you have to first label where the Earth is on its orbit in mid-September and in mid-March. To do this, you need to know that the Earth revolves around the Sun counter-clockwise, as seen from the top, on a diagram like the one on the previous page. So mid-September will be counter-clockwise from mid-June and halfway to mid-December. Mid- March will be counter-clockwise from mid-December and halfway to mid-June.

On the diagram on p.9, label Earth’s locations in mid-September and mid-March, then label the locations of the stars that make up the constellations that are overhead in mid-September and mid- March.

Set your planisphere back to tonight at midnight.

Can you see the constellations that were overhead in mid-September and mid-March?

Yes, though slightly not clear

Where in the sky is your mid-September constellation?

Southern hemisphere

Where in the sky is your mid-March constellation?

Northern hemisphere.

Use the diagram of the Earth, Sun, and stars to explain this – think about the fact that you can see half the celestial sphere at any given time.

The result of the Earth’s motion around the Sun is that over the course of months the stars would change position in the sky in a way that mimics the diurnal (daily) motion even if the Earth weren’t rotating.

To see this, choose Orion, Ophiuchus, or either of the constellations you wrote in Question 34 and locate it so that about half of the constellation is visible on the eastern edge of your planisphere.

What day of the year lines up with 9 p.m.?

274th day

Move the star wheel so that the day one month later lines up with 9 p.m. (For example, Ophiuchus is half visible at 9 p.m. on approximately April 15; for this question, move the star wheel so that May 15 now lines up with 9 p.m.) What happens to the constellation?

The constellation moves from the north western hemisphere to the northern hemisphere.

Continue to move the star wheel one month at a time (always lining up with 9 p.m.), stopping when the constellation is setting and only half visible on the western horizon at 9 p.m. Describe the motion of your constellation.

The constellation moves from the northern hemisphere to the north eastern hemisphere.

What month is your constellation only half visible on the western horizon at 9 p.m.?

June

How many months after it was half visible on the eastern horizon at 9 p.m. is this?

3 months

Does that number of months make sense? (Hopefully so…) Explain why.

Yes, 9.p.m is equated to the 274th day of the year which means the end of the third quarter of the years meaning there are only 3 months left before the end of the year.

5675630-16764000The combination of rotation and revolution of the Earth, however, means that over the course of a year of nights, all of the celestial sphere north of your southern limit (Question 43 of the Introduction to the Night Sky; should be about -55° declination) is visible. I have tried to show this in the diagram to the right; the dark circle represents the celestial sphere, the thin line shows the celestial equator, and the dashed line shows a declination of

-55°.

Based on this diagram, approximately how what fraction (or percent, if you prefer) of the celestial sphere would be visible over the course of a whole year to someone living in Atlanta (or similar latitude)?

Three quarters of the celestial sphere will be visible since Atlanta is in the southern hemisphere occupying a quarter of the hemisphere hence the part visible would be the remain three quarters of the hemisphere.

What if you lived at the North Pole of the Earth? What would your maximum southern declination be? (Don’t overthink this; it’s pretty straightforward.) Over the course of a year, what fraction or percent of the celestial sphere would be visible?

At the north pole, the fraction of the celestial sphere that will be visible would be three quarters of the entire sphere.

What if you lived at the equator? Over the course of a year, what fraction or percent of the celestial sphere would be visible?

At the equator, the fraction of the celestial sphere that is visible is half the sphere.

Now, let’s open Stellarium to confirm what we’ve been doing with the planisphere and watch it in the night sky. Make sure you’re set for tonight at 9 p.m.

To see the diurnal motion of stars, use the fast-forward button on the horizontal menu bar in Stellarium. It is toward the end of the horizontal menu bar and is called “Increase time speed” to make the celestial sphere advance faster through time.

What are at least three circumpolar constellations that you can see? Go ahead and advance the time a full 24 hours and image you could see the stars in the daytime.

What are at least three constellations that were visible at 9 p.m. but have set by 3 a.m.?

Ursa Major, Ursa Minor and Cassiopeia.

What is different about the declinations of the constellations that are circumpolar and those that aren’t? (Remember you can click on a star to get its RA and Dec.)

All the constellations visible at this time are circumpolar hence there is no difference in declinations.

If you were to observe the sky from a location on Earth closer to a pole than where you are now, would you see more or fewer circumpolar constellations? Would you see a larger or smaller number of all constellations (circumpolar and non-circumpolar combined) over the course of a night?

What if you were to observe the sky from a location on Earth closer to the equator than where you are now, would you see more or fewer circumpolar constellations? Would you see a larger or smaller number of all constellations (circumpolar and non-circumpolar combined) over the course of a night?

At a location closer to a pole, I a more likely to see fewer circumpolar constellations since I my view will be obstructed by the close proximity of the pole. On the other hand, circumpolar constellations are more visible at the equator.

Now let’s focus on Ursa Minor (the Little Dipper part). Advance time over 24 hours and watch the position of both Polaris and the constellation overall. Describe how the position and orientation of the constellation changes over 24 hours.

Draw a sketch showing this. Let the dot below be Polaris; show the position and orientation of the Little Dipper every 3 hours starting at 9 p.m. You should have noticed that Polaris does not move significantly over the course of the night (technically, its declination is just over 89°, so it does make a tiny circle in the sky, but it’s not very noticeable when observing with the naked eye rather than a telescope), so you can use the same dot for Polaris for the sketches at each time. Label your sketches with the times.

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Now observe the Litte Dipper at the same time of the evening for an entire year. Describe its change in position and orientation throughout the year. To show the annual motion of the stars, go back to the Date and Time window on the vertical menu bar and use the up arrow above the month number to advance through the year. The time setting remains the same. You may want to click and drag down the Date and Time window so it doesn’t block your view of how the constellations change.

Next, draw a sketch showing this. Let the dot again be Polaris; show the position and orientation of the Little Dipper every 2 months, always at 9 p.m. Label your sketches with the months!

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Finally, let’s see how the planets change with time. In the Introduction to the Night Sky lab, you should have found that Venus is currently visible in the evening sky. Set the time to tonight at 9 p.m., then watch Venus’ position relative to nearby stars over the course of the night.Describe Venus’ motion and whether or not its position changes dramatically with respect to the nearby stars.

Venus is the slowest moving planet. It only moves once every 243 days hence its position is not going to change.

Set the time back to tonight at 9 p.m. Note Venus’ position relative to nearby stars, then change the date to a week from now, then advance another week, and another, and another. What do you see about Venus’ motion relative to the stars?

Venus is the slowest moving planet. It only moves once every 243 days hence its position is not going to change.

Because Earth and Venus both orbit the Sun (and at different speeds, as we will see in a later lab), our position with respect to the Venus keeps changing continuously. The same is true for the other planets as well. Therefore, we see the planets in front of different back drops (the incredibly distant, hardly changing patterns of stars) depending on where we and they are in our orbits of the Sun. In fact, the term “planetes” means “wanderers” in Greek. The study of the motion o

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