Celestial equator

1. Celestial equator • Divides the sky into northern and southern hemispheres. • If earth’s equator is projected out into space, celestial equator can be obtained

2. Ecliptic • It is apparent annual path of the Sun as projected onto the celestial sphere.

3. Equinoxes and Solstices • Ecliptic and celestial equator intersect only at two points : Equinox (Latin word for “equal night”). • Vernal (Latin word for spring): March 21. • Autumnal: September 23. • Solstice (Latin word for “solar standstill”). • Two other significant locations on the ecliptic. • Northernmost point: Summer solstice (June 21). • Southernmost point: Winter solstice (Dec. 21).

4. Latitude and longitude

5. Celestial coordinates • Declination: Angular distance north or south of the celestial equator. • Declination corresponds to latitude. • Measured in degrees, arcminutes, and arcseconds.

6. Celestial coordinates • Right ascension: Angular distance from the vernal equinox eastward along the celestial equator. • Right ascension corresponds to longitude. • Measured in hours, minutes, and seconds.

7. Altitude and azimuth • Altitude: The angular height of an object above (0 to +90 degrees) or below (0 to – 90 degrees) horizon. • Azimuth: The angle along the horizon, with zero degrees corresponding to North, and increasing in a clockwise fashion. Thus, 90 degrees is East, 180 degrees is South, and 270 degrees is West. Using these two angles, one can describe the apparent position of an object (such as the Sun at a given time).