Why the observations of field astronomy do not involve the measurement of declination and right ascension ?
- ABecause the altitude and azimuth are constantly changed due to the motion of the celestial bodyCorrect
- BBecause the stars do not occupy fixed positions on the celestial sphere
- CBecause the distance of the sun from the earth is variable
- DBecause of the obliquity of the ecliptic
Solution
Concept
Coordinates used in field astronomy
Step-by-step solution
1. Declination and right ascension of a star are tabulated in the almanac and are fixed, so they need not be measured. 2. What the observer measures are the horizontal coordinates (altitude and azimuth) with time, which keep changing with the motion of the body.
Explanation
Field observations are made in the horizontal system. Altitude and azimuth change continuously, and their values with time are used with the almanac data to find the latitude, longitude and azimuth.
Why the other options are wrong
Stars do occupy nearly fixed positions on the celestial sphere (b is false), and the sun's distance and the obliquity are not relevant reasons.
Shortcut
Almanac gives δ and RA; field gives altitude, azimuth, time.
Correct answer: A) Because the altitude and azimuth are constantly changed due to the motion of the celestial body