Note 1: The Night Sky

Page 1

There are two different coordinate systems:

1. Right Ascension-Declination

Right ascension is analogous to longitude and is measured in hours, minutes, and seconds, from the point of vernal equinox

Measurements of declination are either positive (north) or negative (south).

Note: 1 hour = 15° of arc (1° arc is equivalent 1° of longitude) and 1 minute = 1/60 of an hour, or 0.25° of arc

Example: Proxima Centauri has a declination of -62°40’ (the negative indicates it is located 62° below the celestial equator) and a right ascension of 14h 29.7m

2. Altitude-Azimuth

Altitude is the angular distance, measured from 0 to 90 degrees, of a celestial object above the observer’s horizon. Azimuth is the compass angle from due N to the location of the celestial object

Page 2

Circumpolar stars do not sit below the horizon

Although some of the circumpolar constellations (for example Ursa Major and Ursa Minor) are visible all year round, others (for example Orion and Cygnus) are only visible at certain times of the year.

During the summer night, people in the northern hemisphere see Cygnus, but cannot see Orion since it is behind the sun. However in the winter, people in the northern hemisphere do see Orion since the northern latitudes point towards Orion and the sun sits between Earth and Cygnus.