Correct Answer : Center of mass of the Earth
Explanation : The origin of ECI system is located at the mass center of the earth but does not rotate with the Earth. Thus, this is fixed with the relative stars and is hence inertial.
Correct Answer : 23.4°
Explanation : The angle formed between the Earth’s ecliptic plane/orbit plane and the equatorial plane is 23.4°. This is because of the fact that Earth does not rotate perpendicularly in the orbital plane and is therefore inclined.
Correct Answer : Angle between vernal equinox and ascending node on equatorial plane
Explanation : Right ascension is the angle formed between the vernal equinox vector to the ascending node on the equatorial plane. Angle between ascending node and perigee is known as Argument of perigee, angle between the orbital and equatorial plane is called Inclination and finally the perigee and satellite in orbital plane’s angle is known as True anomaly.
Correct Answer : Points 90 degrees to the east of the X-axis in equatorial plane
Explanation : The Y-axis of the ECI system points to 90 degrees to the east of the X-axis in the equatorial plane. Y satisfies the cross product of Z and X thus completing the right-handed coordinate system.
Correct Answer : Vernal equinox
Explanation : The X-axis of the ECI system points towards the vernal equinox. Vernal equinox marks the first day of the spring in the northern hemisphere (21 March). It is the line joining the Earth and the sun when the sun passes through earth’s equatorial plane.
Correct Answer : Horizontal 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.
Correct Answer : Celestial Horizon
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.
Correct Answer : It is based on the direction of rotation of the earth
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.
Correct Answer : 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.
Correct Answer : East point of the horizon, Zenith & the West 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).
Correct Answer : 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.
Correct Answer : 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.
Correct Answer : Vertical circles
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.
Correct Answer : Altitude, Azimuth
Explanation : To identify the location of any celestial object, the Azimuth Elevation system uses two parameters, Altitude & Azimuth. In the Equatorial Coordinate system, the two parameters used are Right ascension & Declination.
Correct Answer : 180° = West point
Explanation : The North point of the horizon is considered 0° Azimuth. From that point, we measure distance eastwards. So that 90° Azimuth is the East point of the horizon whereas 180° & 270° represents the South point & the West point respectively.
Correct Answer : True
Explanation : The North point of the horizon is considered 0° Azimuth. From that point, we measure distance eastwards. The value of Azimuth increases as we go towards east from 0° point. So that 90° Azimuth is the East point of the horizon whereas 180° & 270° represents the South point & the West point respectively.
Correct Answer : It is the angular distance measured from the North point of horizon
Explanation : Azimuth is the second parameter of the Azimuth Elevation system & is defined as the angular distance measured from the North point of horizon in the horizontal direction. It is analogous to the longitude system of the Earth & is measured in degrees.
Correct Answer : 0° = Celestial Horizon
Explanation : 0° Altitude represents the Celestial Horizon as the vertical distance from the horizon is zero. The angular distance is measured in degrees and not in hours. Above the horizon is considered as positive sign & below the horizon is considered as negative sign. So the Zenith can be represented as +90° whereas the Nadir is represented as -90° altitude.
Correct Answer : It is the angular distance of a point from the horizon
Explanation : Altitude is the angular distance of a point from the horizon in vertical direction. So it is analogous to latitude system of the Earth & it is measured in degrees. Above the horizon is considered as positive sign whereas below the horizon is considered as negative sign.
Correct Answer : Altitude = +20° & Azimuth = 90°
Explanation : Azimuth is defined as the angular distance measured from the North point of horizon in the horizontal direction. As the star lies on the vertical circle passing through East point, it represents the Azimuth value. East point of horizon has 90° Azimuth. So the Azimuth value of the star is 90°. Altitude is the angular distance of a point from the horizon in vertical direction. As the star is located at 20° celestial latitude, the Altitude is considered as +20°.
Correct Answer : Heliocentric 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.
Correct Answer : 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.
Correct Answer : Ecliptic 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.
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.
Correct Answer : It is taken as positive towards the Celestial South Pole
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.
Correct Answer : It is constant for a fixed object
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.
Correct Answer : 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.
Correct Answer : In the direction of the Vernal Equinox
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.
Correct Answer : In the direction of the Winter Solstice
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.
Correct Answer : In direction of the Celestial North Pole
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.
Correct Answer : Angular rotation
Explanation : The coordinate transformation changes only the basis of vector, the rest i.e. length and direction after coordinate transformation remains the same.
Correct Answer : (-x, y)
Explanation : When a point has to be reflected across the y-axis, the y value stays the same and the x value becomes opposite of what it is.
Correct Answer : Orthogonal matrix
Correct Answer : 3
Explanation : The coordinate transformation from XYZ frame to X’Y’Z’ frame is carried out by a single rotation about one axis. The other two axes remain in the same plane. But to transform from the ECI coordinate system to the perifocal system, there have to be successive 3 rotations. The first rotation is about Z axis, second is through X’ axis and the final rotation is bout Z’’ axis.
