What are RA and DEC? (Intermediate)

Please explain RA & Dec using the example Persius RA 3h 5m 38s Dec 48 50 2". It is my understanding the Ra is measured in hours, min & seconds. The Dec is measured in degrees but is the 50 & 2 measured in Ft. & inches & what does it mean? Please break it all down to me so I may better explain it to my son. Thanks for your help.

RA (right ascension) and DEC (declination) are to the sky what longitude and latitude are to the surface of the Earth. RA corresponds to east/west direction (like longitude), while Dec measures north/south directions, like latitude.

RA is indeed measured in hours, minutes, and seconds. This is because as the Earth rotates, we see different parts of the sky throughout the night. What this means is that if an object at, say 3h 5m 38s, is overhead now, in an hour from now an object at 4h 5m 38s will be overhead, and so forth. 0 hours right ascension is by convention the right ascension of the sun on the vernal equinox, March 21. So your object is 3h 5m 38s east of the vernal equinox.

Declination is measured in degrees, arcminutes, and arcseconds. There are 60 arcmin in a degree, and 60 arcsec in an arcmin. The symbols for arcmin and arcsec are the same as for feet and inches. (Although sometimes we draw a little arc over the ' and " signs to tell them apart from the feet and inches signs.) So your object is 48 degrees, 50 arcminutes, and 2 arcseconds north of the celestial equator, which is the origin of the declination system at exactly 0 degrees. The celestial equator is the part of the sky which is directly overhead the equator of the Earth. The north star is at about +90 degrees, while the south pole would be at -90 degrees, just like latitude on Earth.

Declination tells you how high overhead your object eventually will rise. So your object at +48 degrees declination would pass directly over a point on the Earth at 48 degrees north latitude each night. If you were standing at, say, 38 degrees north latitude, the object would reach its greatest elevation (height) 48-38=10 degrees north on the sky from overhead.

This page was last updated June 28, 2015.

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Amelie Saintonge

Amelie is working on ways to detect the signals of galaxies from radio maps.

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