An objects’ position in the celestial hemisphere is measured with Right Ascension (RA) and Declination (Dec).
RA is the celestial equivalent of longitude, measuring an object’s eastward position along the celestial equator, commonly expressed in hours, minutes, and seconds, starting at the vernal equinox (similar to the Prime Meridian on Earth, the spot where the Sun crosses the celestial equator going northward in March), increasing to 24 hours; because the sky rotates 360⁰ in 24 hours, 15⁰ of sky motion equals 1 hour of RA.
Dec is the celestial equivalent of terrestrial latitude, measuring an object’s angular distance north or south of the celestial equator, ranging from -90° at the South Celestial Pole to +90° at the North Celestial Pole, and being 0° at the celestial equator. Dec is measured in degrees (°), minutes (‘), and seconds (“), with 60 minutes in one degree, and 60 seconds in one minute.
The ability to view a celestial object from Earth on the imaginary celestial sphere is determined by one’s geographic latitude, time of day/year, and the object’s celestial coordinates. Earth’s rotation from west to east causes objects on the celestial sphere to appear to rise in the east and set in the west. Due to Earth’s orbit around the Sun, the night side faces different parts of the cosmos throughout the year, making certain objects visible only during specific seasons.
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For example, you will find the Carina Nebula (NGC 3372) in green just up and a bit to the right of middle. It is between RA 10h and 12h (10h 45m to be precise) and at Dec -60⁰ (-59° 52′ to be precise).
