Question

1. Definitions of time. (a) Define local siderial time. (b) Why do we use Universal Time (UT), and not International Atomic Time (TAl)? (c) Why is it useful to use the Julian Date (JD)? (d) How would you go about measuring the Declination (DEC) and Right Ascension (RA) of a star?

Answer 1

1. First Point of Aries is the reference point chosen on the celestial sphere to define the siderial time. The time taken for the First point of Aries to complete a rotation and arrive at the observer's meridian is defined as 1 siderial day. Therefore, Local Siderial Time (LST) is defined as the hour angle of First Point of Aries (also called as Vernal Equinox).

2. Due to the gravitational effects of moon on earth, the length of the day increasing by 1.7 milliseconds every century. Universal time is determined by noting the time taken by some fixed stars to cross the meridian. However, International Atomic Time is independent of the the position of the stars. Thus, using IAT in astronomy requires adding offset of few microseconds every now and then. Therefore, UT is more convenient in astronomy than IAT.

3. When conventional time keeping is used, the time between two observations is to be calculated by accounting for different number of days in each month. To avoid these complicated calculations, astronomers have adopted an absolute day count system based on the Julian calendar, which counts the days passed since Greenwich Mean Noon on 1 January 4712 BC.

4. Right Ascension, is the angle between First point of Aries and the intersection of the meridian through the object and equator.

Declination of a star, is the angle between the equator and the star through the meridian passing the star.