|I yelp astronomy like a sun-dog, and paw the constellations like Ursa Major.|
|-- Mark Twain|
The ancients believed the stars to be fixed upon a great crystalline sphere in heaven. Though we no longer share this view, the celestial sphere concept is still valuable in navigation, and it can be a practical model of ones universe. After all, everyone knows that in about 24 hours, the stars, sun and moon all rise in the east and set in the west. In this lab, you will locate constellations by their coordinates (both celestial and local), and estimate your terrestrial location from them as well.
Three coordinate systems must be defined to predict the height of a star above the horizon: the celestial, based on stellar positions; the terrestrial, using Earth-based positions; and the local system, centered on you, the observer. The three systems are summarized below.
|Celestial coordinates||||Right Ascension is measured east of the vernal equinox in hours, minutes, and seconds of time (0 to 24 hr).|
|||Declination is measured north or south (+/) of the celestial equator in degrees, minutes and seconds of arc (90o to +90o).|
|Terrestrial coordinates||||Longitude is measured east or west of Greenwich, England in degrees, minutes and seconds of arc (180o W to 180o E).|
|||Latitude is measured north or south (+/) of the Earth's equator in degrees, minutes and seconds of arc (90o to +90o).|
|Local coordinates||||Azimuth is measured east of the northern horizon, or your meridian, in degrees, minutes and seconds of arc (0o to 360o).|
|||Elevation is measured up from the horizon in degrees, minutes and seconds of arc (0o to 90o).|
Since the sky appears spherical, astronomers measure all distances as angles. For example, from east to west is 180 degrees and from the horizon to zenith is 90 degrees. Notice that the sky rotates a full 360o in 24 hours, or 15 o/hr. With your arm extended, the apparent length of your fist with thumb raised (a la Fonzie) is roughly 15 degrees or an hour. Your fist alone (across the knuckles) is about 10 degrees. The full Moon, a half degree, is roughly the width of your thumb held at arms length.
To know when an object will cross your meridian (reach its highest elevation, or "transit"), astronomers use the hour angle. This angle measures the difference between the right ascension (R.A.) on your meridian (also known as local sidereal time or LST), and the R.A. of the object of interest. For example, if a star transited 90 minutes ago, its hour angle (H.A.) would be 1h 30m. Similarly, if an object on the equator just rose, its H.A. would be 6h or 18h. One may use
HA = LST RA , or HA = RA(on your meridian) RA.