# Lunar Trajectories Questions and Answers – Synodic Period

This set of Spaceflight Mechanics Multiple Choice Questions & Answers (MCQs) focuses on “Lunar Trajectories – Synodic Period”.

1. What is a synodic period?
b) Time taken for phase angle to be perpendicular to the initial value
c) Time taken for phase angle to be exactly 180 apart from the initial value
d) Time taken for angular acceleration to match the original value

Explanation: When any planet/moon revolves, the time it takes to return to the same angular position from the sun as seen from Earth is called as synodic period. The phase angle returns to its original value.

2. What is the formula to compute the Synodic period?
a) Tsyn = $$\frac{|T_1 – T_2|}{T_1 T_2}$$
b) Tsyn = $$\frac{T_1 T_2}{|T_1 – T_2|}$$
c) Tsyn = $$\sqrt{T_1 T_2}$$
d) Tsyn = $$\sqrt{T_1 – T_2}$$

Explanation: If the orbital time periods of two planets/moons are given as T1 and T2 then the synodic period id calculated using the formula:
Tsyn = $$\frac{T_1 T_2}{|T_1 – T_2|}$$
Where, Tsyn is the orbital period of planet 2 relative to 1.

3. If the orbital period of Earth and Venus is 365.26 days and 225 days respectively, then what is the approximate synodic period?
a) 82,184 days
b) 140 days
c) 425 days
d) 586 days

Explanation: Given, TEarth = 365.26 days, TVenus = 225 days
To calculate the synodic formula, we substitute in the formula:
Tsyn = $$\frac{T_{Earth} T_{Venus}}{|T_{Earth} – T_{Venus}|}$$ = $$\frac{365.26×225}{|365.26-225|}$$ ≅ 586 days

4. What does synodic period help in predicting?
a) Planet’s ephemeris
b) Epoch
c) Almanac
d) Keplerian elements

Explanation: Synodic period has been of high importance among the astronomers in predicting the planet’s ephemeris in advance. It is the time taken by the celestial object to return to the same initial angular position from Sun as seen from the Earth. The term ‘synodic period’ is often used for moon, known as lunar month = 29.531 days.

5. What is Venus’s synodic period?
a) 365.25 days
b) 2922 days
c) 583.92 days
d) 2919.6 days

Explanation: Synodic period of Venus is the time taken for it to be seen from Earth in the same position with respect to Sun. It is known to be 583.92 days which is about 19 months. 365.25 days is the synodic period of the Earth.

6. What is the synodic period of the moon?
a) Half of the time taken for it to complete one orbit around the Earth
b) Time between full moon and new moon
c) Time between successive full moon
d) Time to make phase angle equal 90 deg from initial position

Explanation: Synodic period or lunar period is the time taken between successive recurrences of the same phase. This could be time between two consecutive full moon or new moon. For moon the synodic period is 29.531 days which is an average value.

7. What is the value of synodic period of the moon?
a) 27.1 days
b) 28.3 days
c) 27.3 days
d) 29.5 days

Explanation: The time taken for the moon between two successive phases is 29.5 days. On the other hand 27.3 days is the lunar sidereal which is the time taken for moon to complete a 360 degree orbit around the earth relative to the sun.

8. When does the planet disappear into sun’s glare once within each synodic cycle?
a) Opposition
b) Solar conjunction
c) When it is aligned with the line joining earth and sun
d) Lunar conjunction

Explanation: For outer planets there is a point when the Earth and sun are in a straight line and the planet I at exact opposite part of the sky to the sun. Within each synodic cycle, thus the planet disappears into sun’s glare once at solar conjunction. Half a cycle later, it is visible for much of the night when it reaches opposition.

9. Planet’s closer to the sun such as Mercury and Venus have a long synodic period.
a) True
b) False

Explanation: The planets which are closer to the sun have a shorter orbital time period and have a similar speed compared to the Earth. Thus it takes longer for it to catch up to Earth’s orbital progress in the intervening time.

10. Which of these planet’s synodic period is more than its sidereal period?
a) Mercury
b) Venus
c) Mars
d) Jupiter

Explanation: Mars is the only planet in our solar system whose synodic period is more than its sidereal period. Its sideral period is 687 days whereas its synodic period is 780days. The synodic period is the time taken by the planet or any celestial object to come back to the initial angular position whereas the sidereal period is the time taken by the planet to orbit 360 degrees around the Earth as seen from Sun or any other fixed star.

Sanfoundry Global Education & Learning Series – Spaceflight Mechanics.

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