# Orbital Mechanics Questions and Answers – Orbit Determination – Time Systems

This set of Orbital Mechanics Multiple Choice Questions & Answers (MCQs) focuses on “Orbit Determination – Time Systems”.

1. What is the time between two successive upper transits of the sun across a local meridian called?
a) Apparent solar day
b) Solar day
c) Sidereal time
d) Universal time

Explanation: The time between two successive upper transits of the sun across our local meridian is called an apparent solar day. It differs according to the observer’s location i.e. local longitude. Sidereal time is with reference to the stars. Universal time is the mean solar time at Greenwich meridian.

2. Which of the following is a correct statement about time?
a) Time is not a fundamental dimension in science
b) Time can be calculated by the same method throughout the universe
c) The main purpose of time is to define with precession, the exact moment of a phenomenon
d) The epoch designates a particular instant described as a day

Explanation: The correct sentence is that ‘the main purpose of time is to define with precession, the exact moment of a phenomenon’ because Time is a fundamental dimension in science, it cannot be calculated by the same method throughout the universe and the epoch designates a particular instant described as a date.

3. A solar day comprises of ______
a) 12 hours
b) 24 hours
c) 23 hours 56 mins
d) 24 hours 4 mins

Explanation: A solar day is the time required for the sun to return to the same position overhead. As observed from earth, the duration amounts to 24 hours.

4. What is the reference for the Sidereal time?
a) The Sun
b) The centre of the Celestial sphere
c) The Moon
d) The Stars

Explanation: The Stars is the reference for the sidereal time because their relative location doesn’t change much even after a year owing to its large distance from the Earth.

5. From the figure, the earth rotates in the Sidereal day and the solar day by_____

a) 360°, 360°
b) 360°, 360.986°
c) 360.986°, 360°
d) 360.986°, 360.986°

Explanation: The duration of a Sidereal day is 23 hours 56 mins and the earth rotated by 360°. The duration of the Solar day is 24 hours and the earth rotates by 360.986°. A Sidereal day is shorter than a Solar day.

6. Local sidereal time of a site is the time elapsed since the local meridian of the site passed through the vernal equinox.
a) True
b) False

Explanation: Local sidereal time is measured by the duration between two successive transits of the vernal equinox over the local meridian. The vernal equinox is the point common to the celestial equator and the ecliptic plane. Ecliptic plane is the path of sun’s travel around the earth, with earth fixed at the centre of the celestial sphere.

7. Which is not a time system?
a) Julian Day
b) Atomic time
c) Universal time
d) Polar time

Explanation: Out of the options, Polar time is incorrect. Others are different types of time systems. The Julian day is the number of days since noon UT on January 1, 4713 BC. Atomic time defines one second as the time it takes a Cesium-133 atom at the ground state to oscillate exactly 9,192,631,770 times. Universal time is determined by the sun’s passage across the Greenwich meridian, which is 0° terrestrial longitude.

8. The system measured by the sun’s passage across the Greenwich meridian is called _____
a) Universal time
b) Apparent Solar time
c) Sidereal time
d) Julian day

Explanation: Universal time is the mean solar time at the Greenwich meridian. It depends on the rotation speed of the Earth. It denotes the mean duration from one noon to the next noon, at Greenwich meridian.

9. Julian day number is the number of days since _____
a) December 1, 4713 BC
b) January 1, 2000
c) January 1, 4713 BC
d) December 1, 1900

Explanation: The Julian day number is the number of days since the noon on January 1, 4713 BC as per Universal Time. It counts the days elapsed since the mentioned date & finds use in astronomy and software purposes. Julian Day Number (JDN) is an integer value denoting each solar day passed.

10. Which of the following statement is incorrect about Julian day?
a) The Julian day count is uniform and continuous and does not involve leap years or different numbers of days in different months
b) The number of days between two events is found by simply subtracting the Julian day of one from that of the other
c) The Julian day begins at noon rather than at midnight so that astronomers observing the heavens at night would not have to deal with a change of date during their watch
d) Julian day is more useful for people resident on the Earth

Explanation: The Julian day is mainly used by the astronauts and astronomers dealing with outer space to keep track of time. In these cases, the universal time is taken as a reference. People living on the surface of the earth prefer other time systems like the Universal time or the Local time.

11. Julian Day Number (JDN) can be calculated as _____
a) day = $$\frac{given\, month – 14}{12}$$ = d
b) year = given year + d – 4800 = y
c) month = given month – 12d + 3 = m
d) JD = d + $$\frac{153m + 2}{5}$$ + 365y + $$\frac{y}{4} – \frac{y}{100} + \frac{y}{400}$$ + 32045

Explanation: The Julian day number is given by the formula
JD = d + $$\frac{153m+2}{5}$$ + 365y + $$\frac{y}{4} – \frac{y}{100} + \frac{y}{400}$$ + 32045
Where d, y & m can be calculated as
day = $$\frac{14 – given\, month}{12}$$ = d
year = given year – d + 4800 = y
&month = given month + 12d – 3 = m.

Sanfoundry Global Education & Learning Series – Orbital Mechanics.

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