To pinpoint the positions of objects on the celestial sphere, astronomers use a system of coordinates similar to latitude and longitude. Right ascension (RA) is the equivalent of longitude, declination (dec.) the equivalent of latitude. Declination is the easier to understand, as it is graduated like terrestrial latitude in degrees, from 0° at the celestial equator to 90° at the celestial poles. Right ascension, by contrast, is divided into 24 hours, because the celestial sphere appears to rotate once in that time. RA runs from west to east, the direction in which the Earth turns.
star's angle of declination (45°) above celestial equator, first point of Aries (vernal equinox point) is the origin for right-ascension measurements north celestial pole celestial meridian - the line of 0° right ascension star position celestial equator.
north celestial pole star's angle of declination (45°) above celestial equator, first point of Aries (vernal equinox point) is the origin for right-ascension measurements celestial equator.
star position star's angle of right _ascension (1 hour or 15°)
recording a star's position
The star in this diagram has a declination of 45° and a right ascension of 1 hour, or 15°. Right ascension is the angle of the object measured from the celestial meridian, which intersects the celestial equator at the first point of Aries.
star's angle of right _ascension (1 hour or 15°)
highway in the sky
The ecliptic is the highway that the Sun appears to follow against the background stars as the Earth orbits it every year. If the Earth's axis were perpendicular to its orbit, the ecliptic would coincide with the celestial equator. In practice, the Earth's axis is tilted at 23.5°, so the ecliptic is angled by 23.5° relative to the celestial equator. The ecliptic cuts the celestial equator at two points, termed the equinoxes. When the Sun is at these positions, in late March and late September, day and night are equal in length the world over. Right ascension begins from the position of the March equinox, in the same way that the Greenwich meridian marks the zero point of longitude on Earth.
All the planets orbit in a narrow plane remaining close to the ecliptic. The band of constellations through which the ecliptic passes is known as the zodiac.
Earth's tilt causes seasonal differences. The Sun's paths across the sky are shown here as seen from mid-northern latitudes at the summer and winter solstices (top, bottom) and the equinoxes (middle).
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