Introduction to the Night Sky
The goals of this lab are to give you an introduction to what you can see in the nighttime sky, how we label the locations of objects on the sky, and prepare you to make nighttime sky observations.
Before we get started, we need to define some terms. I’m sure you have used the word “constellation” before. However, astronomers term what you probably call a constellation an “asterism”, and mean something a little bit different for the word constellation.
Look up the word “asterism”. What does it mean?
It means a pattern of stars that is not a constellation.
Two asterisms you may be familiar with are the Big Dipper and Orion. In the case of the Big Dipper, the stars that make the dipper are part of a larger constellation, Ursa Major (Big Bear). (The Little Dipper is part of the Little Bear, Ursa Minor.)
Draw a quick sketch of the Big Dipper.
4090416-160871The diagram to the right shows the constellation Ursa Major as an astronomer thinks of it.
Do you see the Big Dipper in there?
Yes, I do see the Big Dipper
Do you see a bear?
Yes, I do see the bear.
Dubhe is on the bear’s back, Muscida is its nose, Talitha is a front foot, Tania Australis and Borealis are one back foot, and Alula Australis and Borealis are the other back foot. Alioth, Mizor/Alcor, and Alkaid are the bear’s tail.
Yes, the bear has a long tail. At least in mythology. The bears that became Ursa Major and Ursa Minor had normal, short tails until
someone (Jupiter) grabbed them by their tails and swung them into the heavens. Getting pulled on so much stretched out their tails. Or so the story goes. Other cultures actually saw this as a raccoon
(which does have a tail, or bear cubs that were following mama bear along. Arab myth has what we think of as the bowl of the dipper as a coffin and the “handle” are people in mourning following along.
So, asterisms are the shapes that we draw using the stars that inspire stories of all sorts. (If you’re interested in more, look for the book Beyond the Blue Horizon by E. C. Krupp.) Constellations, on the other hand, are technically whole regions of sky that contain the asterism. Every bit of sky is included in one of 88 constellations, like every bit of Georgia is included in one of its 159 counties. Sometimes when you’re asked about constellations, we mean the shape you see, and sometimes when you’re asked about constellations, we mean a region of sky. I think it will be clear to you when you need to actually answer a question.
Say, for example, I ask you to draw Orion. Which of the below would I expect you to draw?
Hopefully you answered the one on the left. And your drawing might not even have that much detail
– it’s pretty hard to see Orion’s shield and his upraised arm unless you’re in a reasonably dark place.
But if I asked you what constellation M78 was in, then I’m thinking more about the region of space labelled as Orion. Make sense?
All right, now we’re ready to start with our actual activities.
Print out the two-page document “Planisphere for the Northern Hemisphere” on iCollege and construct it as explained on p. 2. If you have any trouble or aren’t sure what to do, watch the short video on iCollege. This is for temporary use (though you can certainly use it to help with your star and planet observations later in the summer), so you don’t need to use cardstock or paste the pages onto a file folder. However, it’s going to be difficult to visualize what is being discussed in these questions without an actual physical (as opposed to electronic) version, so please figure out a way to print this out.
In the second night sky lab (Motions in the Night Sky), you will also need the “Southern Star Wheel”, so you may want to go ahead and print it out now as well if you are having to make special arrangements to get to a printer.
A planisphere – or star wheel – is basically a star chart that allows you to only see the part of the sky at one time. That’s why there is a cutout that shows you what can be seen at a particular date and time while blocking other stars and constellations. If you were looking at the sky from someplace besides the latitudes of 30 to 45 north (which includes most of the continental US), you would need a different planisphere because different parts of the celestial sphere can be seen. (“Planisphere” comes from the same root word as “planetarium”, a modern word technically meaning “a place for planets”, even though you don’t always see planets when you go to a planetarium… It’s just cooler- sounding than “night-sky-arium”. Probably should have been called a “celestarium” and “celestisphere”…)
Which leads to the next question – what is the celestial sphere? Go to Section 2.1 of OpenStax Astronomy (linked in iCollege), read the Celestial Sphere section, and give a basic definition in your own words.
A little more terminology to think about before we proceed:
What is your zenith?
It is an imaginary point direct above my specific location in an imaginary celestial sphere.
Imagine you go outside in the late evening, say 9 p.m., and call your friend who lives in France. After he or she gets done being annoyed with you (because it’s 3 a.m. there), he/she agrees to go outside as well. Luckily you’ve both got clear skies. When you look at your zenith, do you see the exact same thing as your friend sees when he/she is looking at his/her zenith?
No, I do not see the same thing as my friend sees.
Explain why or why not.
It is because her zenith is different than mine. Her imaginary point in a particular location in an imaginary celestial sphere differs from mine.
What is your horizon?
It is the northern hemisphere.
The edge of the cutout of your planisphere represents your horizon, and whatever is in the middle of the cutout region will be seen at your zenith.
Before we look at the stars, there’s one more thing to note about our planisphere.
If you’re looking at a map, and holding it so that north is at the top of the map, is east to the left or right?
East is to the right.
What do you notice about your planisphere’s directions?
Since I am in the northern hemisphere, all my directions are to the right.
Why is this, you say? Well, when you’re looking at a regular map, you’re interested in what the ground looks like as seen from above. But when you’re using a planisphere, you’re looking at the sky as seen from below.
Hold your planisphere at your zenith. (Make sure you can see the side with the directions and stars…) Face north (or pretend to). Hold your planisphere (still at your zenith) so that “Northern Horizon” on the planisphere is in the same direction as you’re facing.
If north is in front of you, is east to your left or right?
East is to the right.
A-ha… It matches which way east is on the ground now!
Now, let’s figure out what will be in our nighttime sky tonight at 9 p.m. Pinch the star wheel part of your planisphere so that today’s date is near your thumb. Place the star wheel in the holder so that your thumb – today – is near 9 p.m.
What constellations/asterisms are visible in the planisphere window whose names you recognize?
Cassiopeia and Polaris.
What constellations/asterisms are visible in the planisphere window whose names you DON’T recognize?
Draco and Cepheus
What two constellations are closest to the center of the planisphere window? These constellations would be closest to your zenith if you were to go out and observe at this time.
Canis Major and Orion
What two constellations are nearest to the eastern horizon?
Orion and Virgo
What two constellations are nearest to the western horizon?
Big Dipper and Ursa Major
What two constellations are nearest to the northern horizon? (Make sure you choose constellations that are completely visible, not ones that are partially below the horizon.)
Ursa Minor and the Little Bear.
Now, when you actually go outside and look, you probably won’t see nearly as many stars as are on your planisphere, if you’re observing from anywhere near Atlanta or anyplace with many street lights…
2882900542990The picture below is from a different planisphere star wheel that shows only the stars you can see when you are in the city. Probably you will be able to see a few more stars than this, but no guarantees.
Which constellations are you most likely be able to see?
Pheonix and Lupus
Mark these constellations on your planisphere somehow.
Let’s practice identifying constellations. Open the “Summer Sky Map”. While this map is perfect for 9 p.m. in late July, it is more like midnight in mid June, so you may want to adjust your planisphere to better match up.
Determine which constellations are the ones labeled A, B, and C. Ignore D and E for a moment; these are individual stars and we’ll come back to this.
Constellation A: Orion
Constellation B: Ursa Major
Constellation C: Canis Minor
Next use your planisphere and the summer skies map to identify at least four constellations, especially those you are likely to be able to see during the summer, on the star field below. (Hint: it’s a little more “zoomed in” and doesn’t show things that would be near your horizon.)
Use your planisphere and identify at least seven constellations, especially those you are likely to be able to see during the summer, on the star field below. Ignore the gray strip through the picture – it is supposed to represent the Milky Way, but you won’t see that unless you are in a very dark location. In this diagram, you are seeing a fairly wide field of view, so there are more constellations.
We will come back and discuss the motions of these constellations over the course of a night and over the course of a year in the next lab. Before that, however, we need to look more closely at the celestial sphere and how we label the locations of celestial objects.
Go to the “NASA – Solar System Exploration – Reference Systems” page linked on iCollege. Review the Celestial Sphere section, and replicate the illustration of the celestial sphere in the space below. Make your diagram large enough that we can add more to it.
Hopefully you noticed that the Earth isn’t straight up and down in the diagram. Why not, you may ask? Because it’s tilted relative to the ecliptic. And what is the ecliptic? There are two different ways to write a definition of the ecliptic, but both are actually the same. A quick search for a definition of “ecliptic” would probably yield you an answer like “the Sun’s apparent path over the course of a year”. Why does the Sun move in the sky over the course of a year? Because the Earth is orbiting it and our perspective changes over months! We’ll draw diagrams of this in the next lab.
Over the course of a year, the Sun moves higher and lower in the sky. This is because the Earth’s rotation axis is tilted with respect to the plane of the solar system. In the Introduction to ASTR 1010L and the Solar System, you looked at a diagram of the solar system as seen “from the top”. If you look “from the side” (see the diagram in iCollege), all the planets orbit the Sun in almost the same plane (most of the asteroids and some of the comets do, too, but they’ve had a more chaotic past and so aren’t necessarily exactly in the plane). We define the plane of the solar system as the plane (in other words, the flat two-dimensional mathematical “surface”) of the *Earth’s* orbit around the Sun.
Because the Earth’s equator isn’t parallel to the ecliptic, we say the Earth is tilted.
Look further down the “NASA – Solar System Exploration – Reference Systems” page to the second illustration of the celestial sphere (don’t read the “Declination and Right Ascension” section yet – we’ll do that in a few minutes). Click on the illustration to watch the Earth spin, then add the ecliptic to your diagram above.
One more question before we move on to learning about the coordinate system we use to label the positions of objects in the sky.
You can’t label the zenith on a general celestial sphere like you’ve drawn above. Why not?
Because zenith is an imaginary point in an imaginary celestial sphere hence cannot be represented in drawing.
Go to the UNL Astronomy Downloads page linked in iCollege, and click on “NAAP Labs” for the appropriate operating system of your computer. Once the file downloads, open it. You should get the “Interactives Setup Wizard”.Note – please leave this installed on your computer for the summer; we will use these simulations again!
Go to “3. The Rotating Sky”, then open “Sidereal Time and Hour Angle Demo”. The simulation shows a horizon view of the celestial sphere; the yellow circle that goes “across” the view from east to west is the celestial equator. The celestial poles are also shown. You do not need to be concerned with the yellow or blue semi-circles that are drawn from north celestial pole to south celestial pole.
Change the observer’s latitude to 33.7° to mimic the view from Atlanta.
When the simulation opens, a star has been placed at RA of 4h and a dec of 30°. (RA is short for “right ascension” and dec is short for “declination”. For RA, just say the letters R A, and dec is said with a hard c just like deck.) The next exercises will help you see what the terms right ascension and declination mean.
Change the star’s dec to 15° then to 60°. What about the star’s position on the celestial sphere changes?
A star 15°s above the Western horizon on the Celestial Equator will take one hour to reach the Western horizon and set. At 60°, the star is above Austin’s horizon.
Where is the star when its dec is 0°?
At the celestial equator.
What does it mean if a star has a negative declination?
The star is circumpolar and does not rise above the horizon.
What are the minimum and maximum dec a star can have? Explain.
It ranges from 0 to 90 degrees. Zero degrees is when a star is circumpolar and 90 degrees is the highest altitude a star can have.
Change the star’s RA to 6h. What about the star’s position on the celestial sphere changes?
The star’s position moves from east to west of the horizon.
What are the minimum and maximum RA a star can have?
A star straight overhead has the maximum altitude of 90 degrees.
The “h” in the RA stands for “hours”. Why do you think this is how a star’s RA is measured? How does it mesh with the minimum and maximum RA you found above?
RA is measured in hours, minutes and seconds of time. It is quite similar to the RA I found above as the minimum RA is 0 degrees and the maximum is 90 degrees.
Experiment with changing the observer’s latitude. The location where a star is seen in the sky changes as the observer changes position on the Earth, but do the RA and Dec of a star change as the observer’s location changes? Justify your answer with observations from the simulation.
Yes, different locations have different RA and Dec measurements. For example, a star at the celestial equator will have a RA and Dec of 0 degrees while a star at the highest altitude will have a maximum RA and Dec of 90 degrees.
Move the observer to the North Pole (90° N latitude). What is directly over the observer’s head?
The North Star, Polaris.
Go back to “NASA – Solar System Exploration – Reference Systems” and read the “Declination and Right Ascension” and “The equinoxes” sections. Based on this reading and your experimentation with the simulation, define what right ascension and declination are.
Right ascension (RA): An angular distance of a particular point measured eastward along the celestial equator from the Sun.
Declination (Dec): It is the position on the celestial sphere that the celestial body lies.
RA and Dec are not usually labeled on planispheres. Instead, we must use a sky map or planetarium program to get more information.
Look back at the diagram of Orion on the second page of the lab. Find Bellatrix.
To estimate its declination, note that it is between 0° and +10° but closer to 10° than 0°. Therefore its declination is about +7°. Bellatrix’s RA is between 5 h and 6 h. Because an hour of RA is pretty large, it must be subdivided. Instead of decimals, such as 5.5 h, RA is subdivided into minutes; 5.5 h would be 5 h 30 m. Because Bellatrix’s RA is slightly closer to 5 h than exactly halfway between 5 h and 6 h, lets estimate that its RA is 5 h 25 m.
Using this method, estimate the RA and Dec of the following stars. You will also need to look at the Ursa Major diagram on the first page of the lab.
Dubhe RA: 5 h 30 m Dec:
Rigel RA: 5h 25 min Dec:
Betelgeuse RA:5h 5 min Dec: 20°
Follow the link in iCollege to get to the download page for Stellarium, a free interactive “planetarium” program. Click on the appropriate operating system and download Stellarium. While it is downloading, note that a PDF User Guide is linked in the upper right corner of the download page.
Install Stellarium. You will need to have administrative privileges to install. (The Windows 64 bit version was 170 MB to download and took about 2 minutes to install.)
Move your mouse all the way to the bottom half of the left side of the screen so that the icon bar appears. Click on the top icon (an 8-pointed star; “Location window”). Find your location (the search bar is below the list in the upper right quadrant of the window that pops up) – “Atlanta”, “Dunwoody”, and “Marietta” all work; some other locations in the area did not. Use something close – the exact location within the metro area will not make a difference. Do make sure that after you choose the location (click on the city name, then make sure it appears in the “Name/City” box in the lower right quadrant), the red arrow points to a location on the map in the upper left quadrant that agrees with where you actually are… You can then click “Use current location as default”.
What time (on your watch or phone) is it? Does this match the time displayed in the center bottom of the screen? What do you observe on the initial view?
It is 0800 hours. Yes, the pointer at the bottom of the screen points the 8 star. In the initial view, the time appear to be incorrect.
Open the “Date/time window” (a clock) on the icon bar just below the “Location window” icon. If your initial view is showing daytime, then choose a nighttime hour. If your initial view is showing the night sky, then choose a daytime hour. (The clock is a 24 hour clock rather than using a.m. and p.m.). What do you see?
The initial position of the star changes. It rotates to almost the same position within 24 hours. However, the star is only visible in the night sky.
You can click on any point in the sky to get its right ascension and declination, and, if you click directly on a celestial object, a lot of specific information about the object (right click on the information to close it out). You can change the direction that you are looking into by clicking and dragging with your cursor any point in the sky into a new position. The cardinal directions of your north/south/east/west points are labeled along your horizon.
Take a few minutes to experiment with Stellarium. Choose two brighter stars to click on and note their names, what constellations they are in, and what their RA and Dec are. You may round off the RA and Dec as given in the object information.
The north, south, east, and west points are labeled along your horizon. The downwardly curved lines of Right Ascension are marked across the top of the screen and the horizontally curved lines of Declination are marked along the side of the screen.
Stellarium gives a couple of different RA and Dec for objects. You will see they are fairly similar, but the RA and Dec actually change very slightly with time because the Earth’s spin is precessing like a top wobbling! (Except it wobbles over 26,000 years, so we really don’t notice it…) Choose the “on date” RA and Dec to record for this lab, rounding to the nearest minute (m for RA and ‘ for Dec). As you might expect, there are 60 minutes in each hour of RA, but there are also 60’ (‘ stands for arcminutes) in each degree of declination. We didn’t estimate arcminutes of declination earlier because these divisions are quite small – too small to differentiate by eye on maps.
Star 1: Constellation: Orion RA: 6 h 30 min Dec:4°
Star 2: Constellation: Canis Major RA:3h 30 min Dec:8°
In order to navigate the night sky more easily, you can enable constellation lines and names, a celestial grid with lines of right ascension and declination, the ecliptic, the celestial equator and poles, and more. Move the cursor to the bottom left corner of the screen so that two menu bars show up, one horizontal, one vertical. Fix these button bars so they are always visible by clicking on the tiny black arrows in the bottom left corner so they turn into tiny black squares.
Depending on the time of day you are working on this lab exercise the sky shown by Stellarium may not be dark. Move your cursor over the buttons of the horizontal menu bar so their titles will show up. Click on the Atmosphere button (Sun behind cloud icon) so that it changes from white (enabled) to grey (disabled). This turns off the effect of the atmosphere and the black sky with stars will now be visible. Now click on the buttons labeled Constellations, Constellation Names, and Equatorial Grid to enable these features. This turns their icons from grey (disabled) to white (enabled).
We still need to enable the lines indicating the celestial equator, the ecliptic, and the boundaries of the constellations. Go to the vertical menu bar and click on the “Sky and viewing options window” (the icon with the different celestial objects). Once open, select the tab labeled “Markings”. This tab shows a list of features that can be enabled by clicking the checkboxes next to them. Check the boxes of the following features (the of date version): Equinoxes, Solstices, Equator, Ecliptic, and Celestial Poles. You also have the option to change the color of each of these features if you need to by clicking on the colored box on the other side of the feature. You can close this window. All checked features will now show up.
Finally, enabling the constellation boundary lines will make it easier for us to determine exactly which constellation any celestial objects are in. Go back to the vertical menu bar and click on the Configuration window. Once open, select the “Tools” tab. Under the Planetarium Options panel, check the box for “Show constellation boundary button”. Then close the window. This button has now been added but not yet enabled to the horizontal menu bar to the left of the equatorial grid button. Simply click the constellation boundary button to enable it (the icon will turn from grey to white).
Locate the celestial equator. The celestial equator is labelled and is a slightly lighter blue than the other lines of declination.
Star names are in white and constellation names are in blue. Of course you can click on any star to find out its name, but for this question (and for the next), please choose one of the bright ones that is labelled.
Look at the constellations along the line of the celestial equator until you find a named star. Record below its name, the name of the constellation it is in and whether it is north or south of the celestial equator. Also estimate and record by about how many degrees the star is away from the celestial equator – you can do this based on how far away the star is compared with 10 degrees (each of the lines of declination are 10 degrees apart).
N or S: NorthNumber of degrees from celestial equator: 10°
Next locate the ecliptic (in orange) and look at the constellations along this line for a named star. Record below its name, the name of the constellation it is in and whether it is north or south of the ecliptic. Also estimate and record by about how many degrees the star is away from the ecliptic.
Star: CapellaConstellation: Canis Major
N or S: SouthNumber of degrees from ecliptic: 30°
When you click with the cursor on a point on the ecliptic, a long list of information will be shown about this point, especially, if there is also a celestial object.
For the questions below, find the most northerly and most southerly points along the ecliptic and then click on them. Look for the RA/DEC (on date) line to read off the declination. When you are done with the list of information you can right-click on it to close it out.
Don’t forget you can click and drag the sky around. You may need to turn the horizon off so that Stellarium will show you parts of the sky that aren’t actually visible at the current time. To do this, click the “Ground” icon in the horizontal menu bar so that it turns from white to gray.
What is the maximum (most northerly) declination the Sun will reach?
What is the minimum (most southerly) declination the Sun will reach?
Do these numbers look familiar? Why do you think they have the values they do? (Hint – look at the third paragraph in the “Declination and Right Ascension” section of “NASA – Solar System Exploration – Reference Systems”.
Yes, the values are quite similar. At the solar system, the star with the lowest attitude has a Dec of 0 degrees while the star with the highest altitude has a Dec of 90 degrees.
The point at which the Sun is at its most northern position is called the summer solstice (in the northern hemisphere), and the point at which the Sun is at its most southern position is the winter solstice.
Label the summer and winter solstices on your diagram of the celestial sphere early in the lab.
Follow the ecliptic to the two points where it crosses the celestial equator. Record the RA and Dec of these positions.
Where the ecliptic crosses from south of the equator to north of the equator: RA: 3h 15 minDec: 15°
Where the ecliptic crosses from north of the equator to south of the equator: RA: 6h 45 minDec: 45°
Where the ecliptic crosses from south of the equator to north of the equator marks the beginning of spring in the northern hemisphere, so it is called the “vernal” (or spring) equinox. Where the ecliptic crosses from north to south of the equator is the autumnal equinox.
The vernal equinox is used to define the zero point of right ascension. Label the equinoxes on your diagram of the celestial sphere early in the lab.
Next, let’s focus on the stars and constellations. Use Stellarium to figure out the names of the stars marked D and E in the summer skies map. Round to the nearest minute.
Star D: RigelRA: 9 h 10 minDec: 40°
Star E: VegaRA: 12 h 40 minDec: 10°
Look around and find Vega in the sky.
What constellation is Vega in? What is its RA and Dec?
Find Capella. This time, use the “Search Window” button on the vertical menu bar. Stellarium will pan to Capella and show a list with detailed information of the object.
What constellation is Capella in? What is its RA and Dec?
RA: 18h 36m
Next, search for M51. What is M51? What constellation is it in?
It is a whirlpool of hydrogen gas that creates new stars and forms a galaxy.
Constellation: Canes Venatici
What city are you using for your location in Stellarium?
Turn the horizon back on using the Ground icon. Move the sky around so that you can see the lowest declination possible (hint: it will be negative).
What is the maximum south (most negative) declination you can see from where you are, provided you are in the northern hemisphere (if you are in the southern hemisphere, estimate the maximum north declination)?
Maximum south= -10°
Maximum north= 10°
Use the search feature to find the Large Magellanic Cloud and the Small Magellanic Cloud, two small galaxies very close to the Milky Way. What are their RAs and Decs?
LMCRA: 15h 32 minDec: 37°
SMCRA: 12h 18minDec: 42°
Are the LMC and SMC visible from your current location on the Earth? Why not?
LMC and SMC are not visible in the northern hemisphere.
At what latitude on Earth would you build an observatory from which to see all of the sky? Explain your reasoning.
The best place to make observations is at 0 degrees which is at the equator. At this latitude, it is possible to get and angular view of the sky since the whole of the sky is visible.
You may have noticed that we haven’t mentioned planets, and yet they can be brighter in the sky than any of the stars. The reason is that as they orbit around the Sun, their position changes with respect to the stars. The word “planet” comes from a Greek word meaning “wanderer” because the ancients saw the planets wandering among the stars.
Their names have to do with how they look in the sky: Mercury, the messenger god of the Greeks and Romans (known as Hermes to the Greeks), moves very quickly in the sky because it is close to the Sun and orbits very quickly. Venus, the only planet named for a goddess, has a cyclic pattern in the sky lasting 9 months. Mars, named for the god of war, appears red due to the iron oxide (rust) on its surface. Jupiter, the king of the gods, moves at a stately slow pace and is brighter than any planet except for Venus. Saturn, the father of Jupiter, moves more slowly and isn’t as bright as Jupiter. (The other planets aren’t visible to the unaided eye, in other words, without binoculars or telescope.)
In order to find out what planets are visible at a particular time, a good reference is Sky & Telescope Magazine’s This Week’s Sky at a Glance, linked on iCollege.
You can skim through the top part and look at the diagrams, but you will get the most concise information in “This Week’s Planet Roundup” at the bottom of the page.