4th Day Alliance

For centuries, astronomers have noted that the night sky looks like a huge sphere with stars placed on the inside surface of the sphere. This imaginary sphere is called the celestial sphere.

Astronomers have identified the positions of stars by their position on the celestial sphere. However, you must remember that since a sphere is circular, we must use degrees to find positions and not linear measurements like feet, inches, or miles.

Coordinates on the celestial sphere are very similar to the coordinates on Earth. On the Earth, we use latitude and longitude. The latitudinal lines go "left and right" or east and west around the Earth, and the longitudinal lines go "up and down" or north and south on the Earth. On the celestial sphere, "latitude" is called declination and "longitude" is called right ascension. Also, the celestial sphere has a celestial equator at it's midpoint that wraps around the sphere and "cuts it in half" (just like the Earth's equator).

Coordinates on the Earth are expressed in degrees, but coordinates on the celestial sphere are expressed a bit differently. Declination is expressed in degrees, arcminutes, and arcseconds north (+) or south (–) of the celestial equator. Right ascension is expressed not in degrees, but in hours (h), minutes (m), and seconds (s) of time, from 0 to 24 hours. Apparently, astronomers set up this convention centuries ago because the Earth completes one turn in about 24 hours. So, the celestial sphere (which appears to have its grid permanently printed on) seems to take about 24 hours to complete one turn around the Earth.

Also, there is one more little problem. Since the Earth's orientation in space changes very slowly over time (a process called precession), the position of stars on the celestial sphere will appear to change over time. This is why stellar coordinates are often printed in star atlases with a year, such as 2009.

The brightness of a night sky object is called its magnitude. You'll run into this term all the time, though you may never get used to the manner it which it is used.

Our current magnitude system was created over 2000 years ago by Greek astronomer Hipparchus. He called the brightest stars, "1st magnitude," those that were a little fainter he called "2nd magnitude," and so on and so on. The faintest stars that he could see were called, "6th magnitude." Once people started using telescopes, then fainter stars could be seen causing 7th, 8th, and 9th magnitude stars to be added to the list.

With modern tools, a pair of binoculars will usually show stars as faint as 8th or 9th magnitude and an amateur 6-inch telescope can usually show 12th magnitude stars. The Hubble can see objects at nearly 30th magnitude! This is about 10 billion times fainter than stars visible to the naked eye!

What can make the brightness scale confusing is the fact that it seems to go backwards. Brighter stars have lower numbers and fainter stars have higher numbers. Furthermore, the brightest of all stars actually have negative numbers. For example, Sirius which is the brightest star in the sky is magnitude -1.4. Venus is usually magnitude -4. The full moon shines at magnitude -13 and the Sun is magnitude -27. I know that can be confusing, but "it is what it is." :-)

Since distances in space are so vast, it becomes silly to speak of things in terms of miles, otherwise we would be saying things like "one thousand 24 quadrillion, 3 trillion, 11 billion miles." Astronomers have created 3 new units of distance to use in astronomy:

The Astronomical Unit
The Earth is approximately 93 million miles (150 kilometers) away from the Sun. This distance is now referred to as the astronomical unit. It is 93 million miles. Astronomers use the astronomical unit (a.u.), to describe distances in our own solar system.

A Light Year
A light year is a measure of distance and not time. More specifically, it is the distance that light will travel at it's known speed of 186,000 miles per second (299,997 km/s). This distance is about 5.9 trillion miles (9.5 trillion km). This distance is about 63,000 a.u. One light year is a distance that is very difficult to comprehend. Nevertheless, even the closest star to our Sun, a star in the Alpha Centauri system, is about 4.3 light years away (over 25 trillion miles). The nearest galaxy to our Milky Way galaxy is the Andromeda Galaxy, and it is approximately 2.5 million light-years away!

Parsecs
Professional astronomers have yet one more unit of distance for truly mind-numbing distances. This unit is called the parsec. One parsec is equal to 3.26 light-years. And, a kiloparsec is 1000 parsecs (326,000 light-years) and a megaparsec is one million parsecs (326,000,000 light-years). Yowza!

The technical details behind a parsec is this—one parsec is the distance where a star shows a parallax of one arcsecond against the background sky when the Earth moves one astronomical unit around the Sun.