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Overview

• Become familiar with the two astronomical coordinate systems
• Investigate the differences and use of the two coordinate systems

Here on Earth we have a coordinate system called latitude and longitude that allow us to pinpoint any location on the globe. Of course, this isn't always useful: giving the latitude and longitude of the grocery store won't help most people find it. Hence we also use local coordinates with instructions like "2.3 miles west of my apartment."

Astronomers also use several different coordinate systems for different purposes. The two primary coordinate systems are the equatorial and the alt-az.

The equatorial system is analogous to the latitude and longitude system we use on Earth. To map out these lines, astronomers start by projecting the Earth's north and south poles and the equator onto the sky. Those projections are called the north and south celestial poles and the celestial equator. Lines of Right Ascention (RA) run through the north and south celestial poles, crossing the equator at right angles just like lines of longitude. Lines of declination circle the sky perpendicular to this like latitude lines. In fact, the declination lines match the latitude lines: the celestial equator is at 0° dec, the north celestial pole at +90° or 90° north dec, and the south celestial pole at -90° or 90° south dec. Since Ann Arbor is at a latiude of +42.3° the declination of zenith is +42.3°. Note the fractional degree may be given as a decimal, or in minutes and seconds of arc. There are 60 arcminutes in a degree, 60 arcseconds in an arcminute.

The right ascension is a little harder. On Earth, 0° longitude is measured from Greenwich, England. We set our watches by the motion of the sun (solar time) again using Greenwich as the standard (universal time) so the Sun should transit the meridian at roughly noon every day in Greenwich. The stars however move 4 minutes faster (sidereal time), so a star that transits the meridian at noon today will transit at 11:56 tomorrow. If we said 0° RA was over Greenwich, the 0 point would move about 1° every day, and you'd have to calculate a new set of coordinates every day. If astronomers use a point on the celestial sphere instead, the zero point doesn't move and the coordinates stay more-or-less the same. The vernal equinox was chosen to be the zero point. RA can be measured in degrees, however it is more practical to measure it in hours. That way, your local sidereal time is equal to the RA of an object transiting the meridian. For example, if your local sidereal time is 20:15, a star at 20^h 15^m 0^s RA would be on the meridian. This also makes it convenient to figure out how long an object will remain visible. Coordinates are normally given with RA first, then dec: the summer solstice is located at 6^h 0^m 0^s and 23° 30^m

Just as the latitude and longitude aren't always convenient on Earth, the equatorial system isn't always convenient for astronomers. So there is an alternate: the altitude and azimuth system.

The altitude is ideally measured in degrees above the horizon. An object at zenith is at 90°, and object on the horizon is at 0°. However, some observing locations may allow for more or less than a 180° view (e.g. on a mountain top or valley). It that case, we imagine where the horizon should be (90° from zenith) and place 0° along that line. Objects visible below this have a negative altitude. We can also figure out how far below the horizon an object is by knowing how long ago it set or when it will rise. In the field, altitude is usually estimated using your hands for reference. The Moon subtends an angle of 0.5°, which is about the width of your index finger held at arm's length. Three fingers take up about 5°, your fist is about 10°, and if you spread your hand out, from index finger to pinky is about 15° and from thumb to pinky is about 20°.

The azimuth is measured in degrees away from north. The system is the same as the markings on a compass: 90° is due East, 180° is due South, and 270° due West.

Coordinates are usually give with altitude first, azimuth second: the position of the Sun on the summer solstice when it transits the meridian is 65.8° at 180°

As you work on this activity try to keep in mind that one of these coordinate systems is "global", completely independant of your location, wile the other is "local", totally dependant on your location. Many of the questions in the activity are aimed at helping you understand the differences between the two systems, and when it would be appropriate to use each system

Last Modified: 5/11/05