TTHE HE SSKY AND KY AND CCONSTELLATION ONSTELLATION TTOUROUR BASICS HOW TO FIND STARS

In practice there are two methods. If you know where to look for stars in the sky, chances are youll find them provided you know roughly how to go about it. It is sort of similar to trying to find Staten Island on a map. If you look at a map of the United States, chances are youd have problems. But if you knew it was close to New York City, and only 12 miles south of it, youd have a pretty good idea where to look. The pictures of the front page are of Orion. If you knew that Betelgeuse is the brightest top left star of Orion (actually in the arm pit of Orion), youd probably find it relatively quickly

However there is one catch you have to be somewhat familiar with the sky. How do you study the sky? Simple you just spend hours and hours identifying various objects. So there is no way around, youll just have to learn the constellations Okay, so what are constellations? Just as the United States is divided into 50 states, the sky is divided in 88 irregular regions called constellations. Constellations make finding objects in the sky easier. Just as specifying that Staten Island is in the state of New York, Betelgeuse is in Orion. The stars that make up the constellations are not actually physically associated but are just patterns that resemble or honor animals, mythological characters, etc.

The Sky pre-lab 2

Many of the constellation names are Greek in origin (e.g., Orion, Hercules and Andromeda), while others are more contemporary in nature (e.g., Microscopium and Telescopium). The Stars are named using several different systems. The brighter stars all have ancient names, while fainter ones are known by number only (although you can name those stars after someone - as a Christmas present). Apart from using popular star names (like Sirius, or even weirder ones like Zubenelgenube or Zubenesschamali), amateur astronomers use the names of constellations, and designate stars in that constellation by Greek letters. To your right there is popular astronomer chart, while the amateur astronomer chart is below. The brightest star is alpha, a, the second brightest one beta, b, then gamma, g, etc. Thus Sirius, the brightest star in Canis Major, is also known as Alpha Canis Majoris. Sometimes, youll see this abbreviated as aCMa. Similarly, Betelgeuse is called Alpha Orionis (or aOri) and Rigel Beta Orionis (bOri). In the constellation charts the brightest stars correspond to big blobs, while fainter ones correspond to smaller blobs. So here is the good news you will not have to learn all the star names, (well, maybe perhaps the 20 brightest ones) standard abbreviations are fine as long as you know what it means.

There is also another method of finding objects in the sky. You can use the coordinates of the star. This is comparable to specifying the coordinates of Staten Island, which has a longitude of 72 degrees and a latitude of 42o. Coordinates are used by professional astronomers, and you are going to be surprised that it is actually easier. All you need to do is type those coordinates into the telescope computer, and voila the telescope slews there. Simple, eh? But thats only because the telescope computer is intelligent. It knows where all the constellations, where on earth you are (no pun intended), what date and what time it is. But thats not all, it also knows how the stars move throughout the night. Well, guess what you will be learning in this course

The Sky pre-lab 3

PPART ART 1 1 F FINDING THE INDING THE PPOLE STAROLE STAR

Most people will agree that the Polaris is one of the most important stars. There is absolutely nothing special about Polaris, it is a yellowish giant star that happens to be located close to the Celestial North Pole. Polaris is NOT the brightest star. In fact it did not even make the list of the 2- brightest stars in the sky. This however, makes it difficult to find Polaris, particularly at CSI where the sky is very bright. The trick is to identify other stars and then look very carefully at the position where Polaris ought to be.

Fig 1

Here is how to do it:

First locate the Big Dipper and Cassiopeia. The Pole Star is between the Big Dipper and Cassiopeia. Draw an imaginary line between the two stars farthest from the handle of the Big Dipper (these are called the pointer stars) and extend this line roughly 3 times towards Cassiopeia. The pointer stars always point to the Pole star no matter how the Dipper is positioned. The Pole star is actually the tail star of the Little Dipper. Since the Little Dipper has fainter stars, you cannot always make it out, particularly if you are in a big city.

The Sky pre-lab 4

QQUIZ UIZ QQUESTION UESTION # 1# 1 1) Now that you know how to find Polaris, go ahead. Here is cartoon of that part of the sky; without

constellations lines drawn in after all, thats what the sky looks like! Bright stars are represented by big dots, and fainter stars by smaller ones. In the pattern below find the Big Dipper, Cassiopeia, and Polaris. Play the game of connect-the-dots, and identify the remaining constellations.

2) Once you finished this part go outside and find Polaris in the sky. Just as a warning, we are south of New York. Thus from CSI Polaris will be due North so you will have to look at the brightest part of the sky, but you should be able to identify the very brightest stars. One more thing, before even trying to look at the night sky, find your self a dark place, i.e., not underneath a street lamp.

The Sky pre-lab 5

PPART ART II II T THE HE CCELESTIAL SPHEREELESTIAL SPHERE

Imagine the sky to be a dome, and like in antiquity, imagine all stars, galaxies, planets, etc., to be painted on the inside of this dome. You see the EARTH only as far as the HORIZON and to you it looks like a circular flat disk. The point directly above you (the top of the dome) is called the ZENITH. Once you locate and face the POLE STAR, you also know that that direction is NORTH. SOUTH is opposite that (behind you); WEST is to you left and EAST is to your right. The height of the POLE STAR (or any star) above the HORIZON is called ALTITUDE. This is basically an imaginary line drawn from the HORIZON to the POLE STAR. Although it looks like a line, it is actually an angle. Recall that distances on the sky are always measured in angles! Below is an artists impression of the sky. (Remember that this is a perspective drawing; the horizon is actually a circle extending all around you, not an oval.) Please learn and know these terms.

Fig 2

ZENITH The point directly above you.

HORIZON The line in the distance where EARTH and sky meet. It makes the EARTH look like a flat, circular disk.

MERIDIAN The imaginary line in the sky connecting NORTH with SOUTH and passing through the ZENITH.

ALTITUDE The height of a star above your HORIZON. It is an angle.

AZIMUTH The angle along the HORIZON from NORTH to the star. This is also an angle.

The Sky pre-lab 6

TTHE HE AAPPARENT PPARENT MMOTION OF OTION OF SSTARS IN THE TARS IN THE SSKYKY During the night, all stars seem to move on the sky. Only one star does not move. That star is the POLE STAR (Polaris). The other stars rotate counter-clockwise around the POLE STAR. To visualize this rotation, imagine a giant umbrella like below, with the POLE STAR at its center and you at the handle. The Big Dipper and Cassiopeia are drawn on this umbrella. As the umbrella turns, you see the stars move around the POLE STAR in a circular manner.

Fig 3

The Sky pre-lab 7

Now translate this motion onto your dome as shown below. The line from the POLE STAR to you corresponds to the handle of the umbrella - it is the AXIS of the CELESTIAL SPHERE as well as the axis of the EARTH.

Consider the motion of the star from the previous drawing of the dome (Fig. 2 of the upper half of the CELESTIAL SPHERE). Lets figure out how that star will appear to move throughout the sky. Like the Big Dipper and Cassiopeia it will rotate counter clockwise around the POLE STAR in a circular manner (indicated by the arrows). The star rises in the EAST and sets in the WEST, as all objects (sun, moon, planets, stars) do. It reaches its highest point in the sky when it crosses the MERIDIAN (the imaginary line connecting NORTH and SOUTH). This is when the star TRANSITS.

Fig 4

The Sky pre-lab 8

HHOW DO YOU FIND STARSOW DO YOU FIND STARS IN THE SKY IN THE SKY??

Well, you could specify the position of a star in terms of ALTITUDE and AZIMUTH. However, this is messy because ALTITUDE and AZIMUTH change throughout the night. Also, they depend on your position , (i.e., your HORIZON and your latitude) on the EARTH.

Clearly, we need a better method to describe the positions of stars on the sky. Astronomers therefore invented sky charts that are comparable to maps. Maps have a coordinate system you can find any place on EARTH by specifying its LONGITUDE and LATITUDE (the x and y axis). Below you can see what we mean by LONGITUDE and LATITUDE.

The positions of the stars in the sky are described in the same manner as cities on the globe. However, instead of using the word LONGITUDE we refer to it as RIGHT ASCENSION and to the LATITUDE as DECLINATION. This can be seen in the diagram below. The DECLINATION is measured in degrees, just like the LATITUDE. However, RIGHT ASCENSION is measured in hours. This means that 360o in LONGITUDE corresponds to 24 hours in RIGHT ASCENSION i.e., the time taken for the EARTH to complete one rotation around its own axis.

Fig 5

The Sky pre-lab 9

Fig 6

LATITUDE The angle from the EARTHs equator to a place on EARTH.

LONGITUDE The angle along the EARTHs equator from the PRIME MERIDIAN to the MERIDIAN (NORTH-SOUTH line) of the city.

RIGHT ASCENSION The angle along the CELESTIAL EQUATOR from the zero point to the MERIDIAN (NORTH-SOUTH line) of the star.

DECLINATION The angle from the CELESTIAL EQUATOR to the star.

CELESTIAL EQUATOR An imaginary circle, similar to the EARTHs equator, but drawn onto the CELESTIAL SPHERE. It is perpendicular (i.e., 90o) to the axis.

CELESTIAL NORTH POLE The point of the CELESTIAL SPHERE which corresponds to the EARTHs NORTH POLE. This point almost coincides with the position of Polaris (it is 1o off).

CELESTIAL SPHERE The imagined sphere surrounding the EARTH on which all celestial objects seem to be drawn.

The Sky pre-lab 10

QQUIZ UIZ QQUESTION UESTION #2#2 1) Answer the following questions:

a) What do you call the angle between your HORIZON and the POLE STAR? ___________

b) What do you call the angle between the CELESTIAL EQUATOR and the POLE STAR? ___________

c) How big is this angle (in degrees)? ___________

d) How big is the angle between your HORIZON and your ZENITH? ___________

2) Explain the difference between Horizon and Celestial Equator

3) Explain the difference between Zenith and Polaris. Under which conditions are they the same?

4) Explain the difference right ascension and longitude?

5) What are the units of right ascension and longitude?

6) Explain the difference declination and latitude?

7) What are the units of declination and latitude?

8) Why do we bother with two coordinate systems? In what sense do they differ from each other, and in what sense are they similar?

The Sky pre-lab 11

QQUIZ UIZ QQUESTION UESTION #3#3 Below is a drawing of the CELESTIAL SPHERE.

1) Label the following quantities: (a) N, S, E, W; (b) the NORTH and SOUTH CELESTIAL POLES; (c) your HORIZON; (d) your ZENITH; (e) the MERIDIAN; and (f) the CELESTIAL EQUATOR.

2) There a five stars in the diagram below. Draw the motion of the stars, including an arrow indicating the direction. The path of Star #2 is shown in Figure 4.

3) Indicate which stars (in the diagram below) are always visible throughout the whole year, during every night. Indicate also which stars are never visible to you.

4) Take a red pen and draw the altitude of Polaris (an angle). Then draw the latitude of the observer in the picture below (also an angle).

5) Take another pen of a different color and draw the 90o-angle between the horizon and the zenith. Then draw the other 90o-angle between Polaris and the Celestial Equator.

6) Based on the four angles you just drew in part (4) and part (5) explain why the altitude of Polaris does always have the same value as your latitude.

The Sky pre-lab 12

QQUIZ UIZ QQUESTION UESTION # 4# 4

Have you noticed that the night sky appears to be different if you travel much further north, or south? You still see the same constellations, but they will appear to be at a different location on the night sky. Also, the stars will follow different paths. Check the celestial sphere, follow the star paths and draw them below. Also label:

a) HORIZON and ZENITH; N, S, E, W on the HORIZON b) POLARIS, the N-S AXIS, the CELESTIAL EQUATOR c) Your LATITUDE; the ALTITUDE of POLARIS d) Finally, draw the paths of the two stars

North Pole 90 Oslo 60

Athens 30 Equator 0

The Sky pre-lab 13

QQUIZ UIZ QQUESTION UESTION #5#5

1. Where (at what latitude) was this picture taken? Explain how you arrived at that answer.

2. The shutter of the camera was left open for several hours for how long? Explain.

3. Which direction is the photographer facing? (This is a trick questions.)

The Sky pre-lab 14

PPART ART III:III: TTHE HE SSKY KY DDURING DIFFERENT URING DIFFERENT SSEASOEASONSNS

Have you noticed that different stars are visible during different seasons? Lets figure out why this is. Look at the diagram below. Lets ignore the rotation of the EARTH around its own axis (only for now) and consider the motion of the EARTH around the Sun. Imagine that there are some aliens out there on a planet that is still unknown, but that is orbiting around Arcturus. Arcturus is in the constellation Botes and is visible in the summer. In fact, in the evenings in June it is the brightest star within about 20 degrees of your ZENITH. So if you wanted, you could wave to the aliens. Half a year later, sometime in December, you want to wave at the aliens again but Arcturus is not up. Look at the diagram below, and convince yourself that the sun would blind you when trying to locate Arcturus. You can do this game with any other star too, and the sky appears to change with the seasons. Clearly, the stars do not move but the sky appears to move because of the EARTHs motion around the sun.

The Sky pre-lab 15

Lets only consider stars that are in constellations along the ecliptic. The diagram below is the same as the previous diagram. In June, at midnight, when you look towards the ecliptic, you see that Sagittarius is transiting, but if you want to look at Sagittarius in December, you would have to look through the sun, and youd be blinded.

The Sky pre-lab 16

QQUIZ UIZ QQUESTION UESTION #6#6 The table below summarizes which constellation in the zodiac you would see at midnight.

Month / Date

constellation transiting at midnight

RIGHT ASCENSION of that constellation

Sun would be seen in this constellation

June 21 Sagittarius 18 hours Gemini July Capricorn 20 hours Cancer

August Aquarius 22 hours Leo September 21 Pisces 0 hours Virgo

October Aries 2 hours November Taurus 4 hours

December 22 Gemini January February March April May

Cancer Leo

Answer the following questions:

In March, which constellation would you be able to see best at midnight? ____________

What is the RA of that constellation? ____________

In May, which constellation would transit at midnight? ____________

What is the RA of that constellation? ____________

Today, which constellation is transiting at midnight? ____________

What is the RA of that constellation? ____________

In which constellation would you find the sun today? ____________

What is the RA of that constellation? ____________

Do you get the idea? Explain below how RA correlates with the season.

______________________________________________________________________________

______________________________________________________________________________

_____________________________________________________________________________

______________________________________________________________________________

How many hours in RIGHT ASCENSION does each month correspond to? ____________

And one week corresponds to hour many minutes in RIGHT ASCENSION? ____________

The Sky pre-lab 17

PPART ART IV: UIV: USING SING SSKY KY CCHARTSHARTS As you have seen the stars move during the night, and not only that, during different seasons different stars will be up. We just determined how during which months certain constellations would transit at midnight. However, what if you want to observe transiting stars at 8pm, not at midnight? That would mean that youd those constellations that transit four hours earlier. So if Virgo transits at midnight in March, Cancer transits at 8pm (remember two hours for every constellations).

In principle this sounds simple, but in practice it is almost too much to think about but as always, there are shortcuts to make this process less messy This where sky charts come in handy

Just as we produce maps from the globe, we also produce charts from the celestial sphere. The diagram below shows how these charts relate to the celestial sphere. Imagine that you peel the celestial sphere just like you would peel an orange. Make a cut along the dotted line B, peel off the cap, and then view the cap from above. This corresponds to the SC-2 chart. Similarly take the part of the peel from between the lines A & A, flatten it out, and you get the SC-1 chart.

In particular check out SC-1. The x-axis corresponds to right ascension (RA), and the y-axis to declination (DEC). We said previously, that stars with a RA of 0 hours will be transiting on September 21 at midnight. But lets assume we want to observe at 8 p.m. at night, i.e., 4 hours before midnight. Thus stars with a RA of -4 hours, or with an RA of 20 hours (i.e., 24-4 = 20 hours) will be transiting. Now check the SC-1 chart. On the x-axis you see RA. Find all stars that have an RA of 20 hours. Below 20h on the x-axis you find a date (in a smaller script) - it says September 20. So at 8 p.m. in the evening of September 20, stars with an RA of 20 hours are transiting.

The Sky pre-lab 18

QQUIZ UIZ QQUESTION UESTION #7#7

Using the SC-1 chart answer the following questions:

Write down todays date.

Which stars are transiting at 8 p.m. today?

Which stars are transiting at 8 p.m. on August 21?

Give the name of a star that will transit today at 10 p.m.:

Capella is a very bright star. On which date will it transit at 8 p.m.?

When (which date) will Capella transit at midnight?

At what time, today, will Betelgeuse transit? At what time, today, will the Andromeda Galaxy (M31) transit?

Which constellation is transiting at 2AM in June?

Which constellation is rising at 2AM?

Which constellation is setting at 2AM?

Using the SC-2 chart answer the following questions:

Write down todays date.

Which stars are transiting at 8 p.m. today?

Which stars are transiting at 8 p.m. on August 21?

Give the name of a star that will transit today at 10 p.m.:

Which constellation is transiting at 2AM in June?

Which constellation is rising at 2AM?

Which constellation is setting at 2AM?

The Sky pre-lab 19

UUSING SING TTHE HE FFINDER INDER CCHARTSHARTS Now lets make this even easier, lets only deal with sky chart SC-2. And let extend it to lower declinations, actually beyond the equator. Thats basically what the ** is. However we are going to use a little trick. We are going to say that North is in a particular direction..

Describe hpw to use the finder charts.

The Sky pre-lab 20

PPART ART V: SV: SOME OME DDESERTESERT PPRECESSION RECESSION & E& EPOCHSPOCHS

So now we know how to find stars depending on the time and the date regardless of whether we specify the stars coordinates, or whether we identify it to be a particular star in a constellation. And we even designed a method that makes it easier for us Great!

But take a deep breath, there is one more variable we did not talk about. Polaris is the Pole Star today (at least it is the closest star to the true North), but this will change. If we wait for 13,000 years, The Pole Star will be Vega. This is because of precession. The Earth wobbles around its axis in the same manner as a gyroscope (this will be demonstrated in the lab).

Thus the orientation of the Earths axis in space will change. North, which we define relative to the North Pole on Earth, will sweep out a circle in the sky. It will take 26,000 years for an entire circle. Lets think about the implications. We explained the motions of the stars according to our current system, with Polaris due North, but this coordinate system itself keeps changing with time There are two things we can do: (a) invent a coordinate system, which does not change (and this truly does exist, but hey, dont we have enough coordinate systems already?), or (b) we can keep the current system which probably does not change too much throughout our lifetime (whats 60 years compared to 26,000 years?) as long as we remember that corrections will be needed once in a while Okay then, lets summarize this, to identify a star we have to specify its current coordinates, the right ascension and the declination, and we have to specify the so-called epoch. If you look up the coordinates of stars, you will most likely also see another quantity. In old catalogues youll see the number 1950, and in new ones the number 2000.

Just a final quiz question: So if it takes 26,000 years for the zodiac to be in the same position as the current zodiac, roughly buy how much do the constellations get offset every 2,000 years? Okay, and when was the European/American Astrology invented? Probably around that time so in other words, your horoscopes are off by a whole constellation ah, I knew it, there is yet another scientific reason not to believe in horoscopes. Well, its a belief and you are free to believe whatever you want!

Picture missing.

The Sky pre-lab 21

A CHALLENGE QUESTION

It is March 21st, Spring Equinox. What time is it in the above picture?

The Sky pre-lab 22

Paste the next 3 maps together. All of them are part of SC001.

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The Sky pre-lab 24

The Sky pre-lab 25