# Spaceflight Mechanics Questions and Answers – Ecliptic Coordinate System

This set of Spaceflight Mechanics Multiple Choice Questions & Answers (MCQs) focuses on “Ecliptic Coordinate System”.

1. Ecliptic Coordinate system is also known as _____
a) ECI system
b) Azimuth Elevation system
c) Heliocentric system
d) Selenocentric system

Explanation: The Ecliptic Coordinate system is also known as the heliocentric system as it takes the Sun as the reference. The ECI and the Azimuth Elevation system take the Celestial Equator and the Celestial Horizon as the reference, respectively. The Selenocentric Coordinate system is based on Moon as the reference.

2. Which point is considered as the origin for the Ecliptic Coordinate system?
a) Celestial North Pole
b) Celestial South Pole
c) Center of the Earth
d) Center of the Sun

Explanation: The Ecliptic Coordinate system takes the center of the Sun as the origin. For the ECI system, the Earth’s center is considered as the origin. Celestial North Pole and Celestial South Pole are not taken as the point of origin in the coordinate systems.

3. Out of the following options, which plane is considered as the fundamental plane of the Ecliptic Coordinate system?
a) Celestial Equator
b) Ecliptic Plane
c) Celestial Horizon
d) Earth’s equatorial plane

Explanation: The Ecliptic plane is the fundamental reference plane of the Ecliptic Coordinate system. The Ecliptic plane is the plane representing the Sun’s path of travel around the Earth in the Celestial sphere. In the Celestial sphere, the Earth is located at the center and is fixed. The Sun revolves around the Earth in a pre-defined path, which is known as the Ecliptic plane.

4. The Ecliptic plane is parallel to the Celestial Equator.
a) True
b) False

Explanation: The Ecliptic plane is not parallel to the Celestial Equator. It is inclined to the Celestial Equator because the same side of the Earth doesn’t always face the Sun as the Earth rotates on a tilted axis. To account for that, the Ecliptic plane is inclined to the Celestial Equator.

5. How is the X axis determined?
a) In the direction of the Vernal Equinox
b) In direction of the Celestial North Pole
c) In direction of the Celestial South Pole
d) In direction of the position of the moon

Explanation: The Ecliptic Coordinate system defines its X axis in the direction of the Vernal Equinox. Vernal Equinox is the point of intersection of the Celestial Equator and the Ecliptic plane. When Sun is at Vernal Equinox, all Earth locations experience identical durations of daylight and darkness. Position of moon is not used to define axis in the Ecliptic system.

6. How is the Y axis determined?
a) In the direction of the Winter Solstice
b) In direction of the Celestial North Pole
c) In direction of the Celestial South Pole
d) In direction of the position of the moon

Explanation: The Ecliptic Coordinate system defines its Y axis in the direction of the Winter Solstice. Winter Solstice is the first day of winter. It is the shortest day of the year for the Northern Hemisphere when the Sun is lowest in the sky. This point is used as a reference for the Ecliptic system.

7. How is the Z axis determined?
a) In the direction of the Winter Solstice
b) In direction of the Celestial North Pole
c) In direction of the Celestial South Pole
d) In direction of the position of the moon

Explanation: The Ecliptic Coordinate system defines its Z axis in the direction of the Celestial North Pole. It is chosen as the reference because it is constant in the Celestial sphere.

8. The Celestial Equator and the Ecliptic plane are fixed in position always.
a) True
b) False

Explanation: The Celestial Equator and the Ecliptic plane are not fixed but are slowly moving. This is known as Precession. It is the slight shift of Earth’s axis of rotation due to wobbling of the Earth. It completes a circle in every 26,000 years. So the Ecliptic system is not an Inertial system.

9. Which of the following is not a parameter of the Ecliptic system?
a) Ecliptic longitude
b) Ecliptic latitude
c) Distance
d) Azimuth

Explanation: The Ecliptic system uses 3 parameters to locate a celestial object in the celestial sphere. The three parameters are Ecliptic longitude, Ecliptic latitude and Distance. Azimuth is a parameter used in the Azimuth Elevation system.

10. Which is the incorrect statement regarding the Ecliptic longitude?
a) It is the angular distance of the object along the ecliptic plane
b) It is constant for a fixed object
c) It is measured eastward from Vernal equinox
d) It ranges from 0° to 360°

Explanation: The Ecliptic longitude is defined as the angular distance of the object along the ecliptic plane. It is measured eastward from Vernal equinox and it ranges from 0° to 360°. Because of precession, the value for a fixed object changes by 50.3 arc-seconds per year.

11. Which is the incorrect statement regarding the Ecliptic latitude?
a) It is the angular distance of the object from the ecliptic plane
b) It is taken as positive towards the Celestial North Pole
c) It is taken as positive towards the Celestial South Pole
d) It lies between ±90°

Explanation: The Ecliptic latitude is the angular distance of the object from the ecliptic plane. It is taken as positive towards the Celestial North Pole. The ecliptic plane is referred to as 0° ecliptic latitude. The value increases to up to 90° on both up and down side.

12. Choose the correct label for the markings A and B in the diagram below.

a) Ecliptic longitude, Ecliptic latitude
b) Ecliptic latitude, Ecliptic longitude
c) Distance, Ecliptic latitude
d) Distance, Ecliptic longitude

Explanation: The Ecliptic latitude is the angular distance of the object from the ecliptic plane. In the diagram, it is represented by marking ‘A’. The Ecliptic longitude is defined as the angular distance of the object along the ecliptic plane, measured eastward from Vernal equinox (VE). In the diagram, it is represented by marking ‘B’.

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