# Spaceflight Mechanics Questions and Answers – Coordinate Systems – Azimuth Elevation System

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

1. Azimuth Elevation system is also known as _____
a) Horizontal system
b) Equatorial system
c) Ecliptic system
d) ECI system

Explanation: Azimuth Elevation system is also known as the Horizontal system because the horizontal plane is taken as reference in it. The equatorial system is another type of coordinate system, which uses the celestial equatorial plane as reference. Ecliptic system is another type of coordinate system, which uses the ecliptic plane as reference. Ecliptic plane is the path traversed by the sun around the earth, keeping earth fixed at the centre of the celestial sphere.

2. What is the fundamental reference taken for Azimuth Elevation system?
a) Vernal equinox
b) Celestial Horizon
c) Celestial North Pole
d) Celestial South Pole

Explanation: The reference for Azimuth Elevation system is taken as the Celestial Horizon. Celestial Horizon is defined as the great circle halfway of zenith & nadir, which are the points just above the observer on the celestial sphere & its opposite point on the celestial sphere.

3. For the Azimuth Elevation system, choose the incorrect statement.
a) Horizon is divided into 4 quadrants: North, East, South and West
b) It is fixed & with respect to the observer
c) It is based on the direction of rotation of the earth
d) Earth & the observer are imagined as a point at the centre of the celestial sphere

Explanation: In the celestial sphere, the Earth is imagined to be fixed and rotating. But the Celestial Sphere rotates along with the Earth. So the Azimuth Elevation system is based on the direction of Earth’s gravity. In this system, the celestial horizon is divided into 4 quadrants: North, East, South and West. It is fixed & with respect to the observer. Earth & the observer are imagined as a point at the centre of the celestial sphere.

4. Zenith can be defined as _____
a) Celestial North Pole
b) The centre of the Celestial sphere
c) The celestial South Pole
d) The point of sky overhead the observer, on the celestial sphere

Explanation: Zenith is defined as the point of sky overhead the observer, on the celestial sphere. It can be also described as the intersection point of the celestial sphere & the normal drawn to the celestial horizon. As the celestial horizon depends on the position of the observer on the Earth, it can coincide with the Celestial North Pole & Celestial South Pole, but that is not always the case. Zenith can never be the centre of the celestial sphere because it does not lie on the sphere boundary.

5. Nadir & Celestial South Pole always coincide in the Azimuth Elevation system.
a) True
b) False

Explanation: Nadir is the point opposite to the Zenith, on the celestial sphere. Zenith is defined as the point of sky overhead the observer, on the celestial sphere. As the celestial horizon depends on the position of the observer on the Earth, it can coincide with the Celestial North Pole & Celestial South Pole, but that is not always the case.

6. Celestial Horizon is always the same as the Celestial Equator.
a) True
b) False

Explanation: Celestial Horizon is defined as the great circle halfway of zenith & nadir, which are the points just above the observer on the celestial sphere & its opposite point on the celestial sphere. Celestial Horizon will divide the sky into different parts depending on the geographic location of the observer. The Celestial Equator is the projection of Earth’s equator onto the Celestial sphere. They may coincide in some cases, but it is not always true.

7. On the celestial sphere, what are the circles that pass through Zenith from the celestial sphere called?
a) Vertical circles
b) Latitude
c) Longitude
d) Horizon

Explanation: Vertical circles are the circles on the celestial sphere that passes through Zenith or Nadir from the celestial horizon. They are half circles made on the celestial sphere. Latitudes & Longitudes are the features pertaining to the planet Earth. Horizon is the great circle halfway of zenith & nadir.

8. The Prime Vertical passes through which three points on the celestial sphere?
a) East point of the horizon, Zenith & the South point of the horizon
b) North point of the horizon, Zenith & the West point of the horizon
c) East point of the horizon, Zenith & the West point of the horizon
d) North point of the horizon, Zenith & the South point of the horizon

Explanation: The Prime Vertical passes through the East point of the horizon, Zenith & the West point of the horizon. The vertical circle needs two opposite points on the horizon to connect; otherwise, it will become triangular in shape instead of a half-circle. So the vertical circle cannot pass through East South & North West pair of points. The vertical circle passing through North point of the horizon, Zenith & the South point of the horizon is known as Local Celestial Meridian (LCM).

9. Choose the right names of the lines marked as A & B in the diagram.

a) A = Prime Vertical, B = Celestial Horizon
b) A = Local Celestial Meridian, B = Celestial Horizon
c) A = Celestial Horizon, B = Prime Vertical
d) A = Prime Vertical, B = Local Celestial Meridian

Explanation: The circle A represents the Prime Vertical which passes through the East point of the horizon, Zenith & the West point of the horizon. The plane perpendicular to the Prime Vertical is the Celestial Equator, which is given by circle B.

10. Which of the following statement is incorrect about Local Celestial Meridian?
a) Local Celestial Meridian (LCM) is the vertical circle passing through North point of the horizon, Zenith & the South point of the horizon
b) LCM is used to measure local time in the Azimuth Elevation system
c) Vertical circle from North through Zenith to South point is called the Upper Meridian
d) Vertical circle from North through Nadir to South point is called the Upper Meridian

Explanation: Local Celestial Meridian (LCM) is the vertical circle passing through North point of the horizon, Zenith & the South point of the horizon. It is used to measure the local time. The vertical circle from North through Zenith to South point is called the Upper Meridian and the vertical circle from North through Nadir to South point is called the Lower Meridian.

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