# Lunar Trajectories Questions and Answers – Earth-Moon System

This set of Spaceflight Mechanics Multiple Choice Questions & Answers (MCQs) focuses on “Earth-Moon System”.

1. What is the mass of moon relative to the earth?
a) 1/6
b) 1/50
c) 1/80
d) 1/100

Explanation: The moon’s mass is 1/80 the mass of the Earth. It is 7.35 × 1022 kg compared to the Earth’s mass which is 5.972 × 1024 kg. The gravitational constant on moon is 1/6th that of the Earth.

2. What is the mean distance between center of both earth and moon?
a) 382,400 km
b) 360,000 km
c) 398,600 km
d) 400,000 km

Explanation: Earth and moon revolve around a common center of mass. The mean distance between the centers of these two is 382,400 km. This value can often vary over the course of the orbit so the mean value of 382,400 km is considered. 398,600 km3 s is the gravitational parameter of the Earth.

3. At what location approximately is the center of mass of the Earth- Moon system located from the surface of the Earth?
a) 1/4th of the way
b) 1/2 of the way
c) 1/3rd of the way
d) 3/4th of the way

Explanation: The distance between earth and moon is about 384,400 km and mass of moon is 1/82.30 of the mass of the earth. This leads to the center of mass being placed at 4,671 km from center of the earth which is also 3/4th the way from center of the earth.

4. How many days does it take for both earth and moon to revolve about their common center of mass?
a) 27 days
b) 21.4 days
c) 27.3 days
d) 360 days

Explanation: Earth and moon both revolve around a common center of mass once in 27.3 days. 27 days is the revolution period of moon and 360 days is the revolution period of the earth.

5. What is the reason for moon’s acceleration in the direction of orbital motion?
a) Tidal bulge
b) Solar pressure
c) Shift in center of mass
d) Longitudinal fluctuations

Explanation: Tidal bulge on earth’s ocean surface caused due to moon is carried eastward due to earth’s rotation. This leads to a shift in the center of gravity of earth to the east of the lining which joins earth and moon. This leads to acceleration of moon in the direction of the orbital motion which leads to moon spiraling outward.

6. What is the term used to refer to the center of mass of earth and moon system?
a) Center of gravity
b) Lagrangian point
c) Barycenter
d) Roll center

Explanation: Barycenter is a term used to refer to the center of mass of two or more bodies. In this case it is the earth- moon system. This point lies inside earth about 4,700 km from its center.

7. When does solar eclipse occur?
a) When sun comes between moon and earth
b) When moon comes between sun and earth
c) When the earth comes between sun and moon
d) When moon comes between earth and mars

Explanation: Solar eclipse occurs when moon comes between earth and sun. Moon’s shadow is casted on earth. They usually happen once in every 18 months. These occur at new moon.

8. When does lunar eclipse occur?
a) When sun comes between moon and earth
b) When moon comes between sun and earth
c) When the earth comes between sun and moon
d) When moon comes between earth and mars

Explanation: Lunar eclipse occurs when earth comes between moon and sun. Earth’s shadow is casted on the moon’s surface. Earth, moon and sun are perfectly aligned. There are two types of lunar eclipses- total lunar eclipse and partial lunar eclipse.

9. Eclipses occur every month.
a) True
b) False

Explanation: Eclipses do not occur every full moon or new moon. This is a consequence of moon’s orbit inclination. It is inclined to earth’s orbit by 5 deg thus it is difficult for all sun, moon and earth to align in a straight line.

10. Moon has seasons just like on Earth.
a) True
b) False

Explanation: The moon’s spin axis is approximately perpendicular to the ecliptic plane (Earth’s orbital plane). This leads to moon having no seasons unlike earth. Sunlight always faces one side leaving the opposite dark in constant darkness.

Sanfoundry Global Education & Learning Series – Spaceflight Mechanics.

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