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Declination is one of the two coordinates of the equatorial coordinate system. The other is right ascension. In astronomy, declination is equivalent to latitude. but it is projected onto the celestial sphere. It is measured in degrees north and south of the celestial equator. Points north of the celestial equator have positive declinations, while those to the south have negative declinations. Objects at the celestial equator have a declination of 0°, those at the celestial north pole have a declination of +90°, and those at the celestial south pole have a declination of -90°.
A star lies in a nearly constant direction when it is viewed from Earth, so its declination is nearly constant from year to year, but the right ascension and declination change gradually due to the effects procession of the equinox, proper motion, and annual parallax. The declination of other celestial objects change much more quickly than the stars.
The declination of the Sun is the angle between the rays of the Sun and the plane of the Earth’s equator. The Earth’s axial tilt(obliquity of the ecliptic) is the angle between the Earth’s axis and a line perpendicular to the Earth’s orbit. The Earth’s axial tilt changes gradually over thousands of years. Because this axial tilt is nearly constant, solar declination varies with the seasons. At the solstices, the angle between the rays of the Sun and the plane of the Earth’s equator reaches its maximum value of 23°26′; therefore it is +23°26′ at the northern solstice and ?23°26′ at the southern solstice. At each equinox, the center of the Sun appears to pass through the celestial equator and its declination is 0°. The Sun’s declination is equal to the inverse sine of the product of sine of Sun’s maximum declination and sine of Sun’s tropical longitude at any given moment. At least that is only a little twisted and confusing.
Without proper declination figures, you could not find a celestial body through your telescope. The same can be said about right ascension. You need the correct numbers for both in order to find the object you wish to observe.
We’ve also recorded an episode of Astronomy Cast all about Coordinate Systems. Listen here, Episode 170: Coordinate Systems.