Introduction to Astronomy

Q1:constellationsQ2: Alt/AzQ3: RA/Dec
PDF of class slides

Introduction, Night Sky

Introduction: First, we looked at the syllabus, of course.

Then we constructed a 'census' of the objects and concepts astronomers study. Many of these (planets, comets, asteroids, etc.) will not be discussed in detail in this class---the Universe is a big place with lots of stuff in it. Most of the material about the planets is straightforward enough that you can easily understand it just by reading about it. You don't need me for planets. You need me for cosmology!

So, we begin...

The night sky has long fascinated people everywhere, and likely this is at least a small part of the reason that you are taking this class. The first problem in astronomy throughout time has been to make sure that everyone knows what particular object we are talking about! This is simplified in other sciences, where you can say, "This big, ugly frog that I have right here in my hand, tagged 'BUF2340954'". But in astronomy, of course you can't hold stars in your hand, and you can't pin a label in their ears. But YOU already are familiar with this problem. The very first thing you do when you want someone to look at some thing in the sky is:

0. Point
That's right. This is the 'zeroth' way to indicate a particular object in the sky. Does it work? NO! You've had this experience yourself, and do not need me to elaborate on how very long it takes for your friends to figure out what the heck you are talking about. So, what do you do? You make:

1. Constellations:
You say "See that line of stars, all of equal brightness? There're about four of them, from North to South?" This is the whole point of constellations---so that ancient astronomers could all identify the same stars or planets to one another. They attached stories because it was fun, and also made things easy to remember, and added order to a very confusing world. If you want to know more about constellations, you can buy star charts, or books about constellations, and look into the sky for yourselves. You don't need me to explain these to you, and professional writers tell the stories much better than your average scientist. You can download and print a 'planisphere', which is a type of star map. A Google search on 'how to read a star map' will turn up all kinds of usefulness. Generally, if you want to find stories about a constellation, just do a web search on the name of the constellation, and you will find out far more than you ever wanted to know about not only constellations, but also about WACKOs who believe all kinds of interesting and bizarre things...

2. Horizon Coordinate System:
First, some vocabulary:

  1. Horizon: The line where the sky meets the ground. This is sometimes hard to find, if there are interesting mountains or trees in the way... but if you go someplace with nothing interesting to see, like the middle of the ocean, or Oklahoma, you can easily see the horizon.
  2. Zenith: The zenith is the point directly over your head. Look up. Yes, that's it, right up there. The angle between the zenith and the horizon (through your body) is 90o.
  3. Altitude: The height of an object above the horizon, measured in degrees. An object on the horizon has 0o altitude. An object at the zenith has 90o altitude. An object halfway in between is at 45o altitude. But this coordinate is not enough. You also need to know the direction (N, S, E or W):
  4. Azimuth: The azimuth tells you how far around the object is from North. It is also measured in degrees, and starts at 0o due North. Due East is 90o azimuth, due South is 180o azimuth, and due West is 270o azimuth.
So now you can indicate a place in the sky with remarkable precision. BUT, the location of an object in the sky depends on YOUR location on the planet, and the time of your observations. (Think of the Sun, which is in different places in the sky at different times, and from different locations...) So, the horizon coordinate system is not ideal for astronomical purposes, since an astronomer will have to keep changing their notes depending on the time of day or their location. (Note: you can actually use this to your advantage, if you have a clock. You can find out EXACT location of a star, using a compass and a sextant. The altitude of the star will give you your latitude, and comparing where the star is in your sky with where it would be in Greenwich, England will give you your latitude. Bingo---celestial navigation. This is a great example of using human ingenuity to turn a problem into an advantage! A great book about this is Dava Sobel's Longitude.)

3. Celestial Coordinate System: So-called because it uses celestial points of reference. This system functions much like latitude and longitude on the Earth. Recall from grade school about latitude and longitude. Again, some vocabulary:
  1. Celestial Equator: the line in the sky directly over the Earth's equator.
  2. North Celestial Pole: the point in the sky directly over the Earth's North pole.
  3. South Celestial Pole: Can you guess? Yes, the point in the sky directly over the Earth's South pole.
  4. Declination: This is like latitude, but in the sky. It measures how far North or South an object is from the celestial equator. Polaris is at 90o declination. The south celestial pole (no star there) is at -90o declination. If you think about this for a moment, you will realize that this means that the declination that is directly over your head is equal to your latitude. Any object directly overhead in Ogden (latitude=41o) has declination 41o. Again, this single coordinate is not enough. You also have to know how far 'around' the object is in the sky---i.e. how far East or West it is.
  5. Equinox, Solstice: Over the course of the year, the Sun moves Northward, so that in the summer, it is North of the celestial equator. Then, it moves South again, so that it is South of the celestial equator in the winter-time. In order to do this, it must cross the celestial equator twice each year. The first time is on March 21, and this is called the Vernal Equinox. The second time is on September 21, and this is called the Autumnal Equinox. The day when the Sun is at it's most northward point, is Summer Solstice (June 21). The day when the Sun is farthest South, is Winter Solstice (Dec 21).

    Here's the trick: The equinoxes and solstices are not only times (dates), but also locations in the sky. The location of the Sun on the vernal equinox is also called the vernal equinox. Just now the location of the vernal equinox is in the constellation Aquarius (remember the awful song 'The Age of Aquarius'? This is what it's talking about---the vernal equinox has just recently moved into the constellation Aquarius---this is due to precession, which we will talk about in a few lectures.)
  6. Right Ascension: What's the point? The point is that the vernal equinox is used as the zero point of the East-West measuring coordinate. This coordinate is called Right Ascension, and is measured in hours, minutes and seconds to the East of the Vernal Equinox.
Confused yet? Yes, I thought so. Here's a picture that might help you sort things out. It's a map of the sky, with the preceding points labeled on it. East is on the left in this map (because you are looking up at the sky, instead of down at the Earth), and North is at the top. I just stuck some stars on for interest---they in no way are intended to indicate the locations of actual stars!

  1. The path of the Sun through the sky over the course of the year. This is also called the ecliptic.
  2. The celestial equator.
  3. The autumnal equinox (notice that the Sun is heading South at this point).
  4. The vernal equinox (notice that the Sun is heading North at this point).
  5. The summer solstice (the Sun comes to North 23.5 declination at it's northernmost point).
  6. The winter solstice (the Sun comes to South 23.5 declination at it's southernmost point).
  7. The direction of the Sun's motion through the sky.
What's the point? Most star maps are marked out in RA and Dec. So if you go buy one, that is what you will see. Also, this is how astronomers finally solved the problem of making an unmistakable grid on the sky, so that we can pinpoint not only objects you can see with your eye, but also those too faint to see without a telescope.
Concept Question 1:
Why don't professional astronomers use constellations to find things in the night sky?
  1. So astronomy will seem more mysterious.
  2. They are trying to be politically correct.
  3. Constellations are large compared to the "field of view" of a telescope.
  4. Constellations do not cover the whole sky.

Concept Question 2:
Suppose a star is located halfway between the horizon and the zenith in the South-West. What are its altitude and azimuth?

  1. 90o, 270o
  2. 45o, 225o
  3. 225o, 45o
  4. 90o, 225o

Concept Question 3:
You are observing in Ogden, and you notice a star directly overhead. Looking at your star chart, you find its coordinates:
R.A.= 5 h 30 m
Dec = 40o
What coordinates will be directly overhead in two hours?

  1. R.A.=3 h 30 m
  2. R.A.=7 h 30 m
  3. R.A.=3 h 30 m
  4. R.A.=7 h 40 m