4. Define Sensible Horizon and Visible Horizon. It is a circle in which a plane passing through the point of observation and tangential to the earth’s surface (or perpendicular to the Zenith-Nadir line) intersects with celestial sphere. The line of sight of an accurately leveled telescope lies in this plane. It is the circle of contract, with the earth, of the cone of visual rays passing through the point of observation. The circle of contact is a small circle of the earth and its radius depends on the altitude of the point of observation. 5. What is meant by Vertical Circle, Observer’s Meridian and Prime Vertical? A vertical circle of the celestial sphere is great circle passing through the Zenith and Nadir. They all cut the celestial horizon at right angles. The Meridian of any particular point is that circle which passes through the Zenith and Nadir of the point as well as through the poles. It is thus a vertical circle. It is that particular vertical circle which is at right angles to the meridian, and which, therefore passes through the east and west points of the horizon. 6. Define the terms: Latitude (θ) and Co-latitude (c). Latitude (θ): It is angular distance of any place on the earth’s surface north or south of the equator, and is measured on the meridian of the place. It is marked + or
– (or N or S) according as the place is north or south of the equator. The latitude may also be defined as the angle between the zenith and the celestial equator. The Co-latitude of a place is the angular distance from the zenith to the pole. It is the complement of the latitude and equal to (90°-θ). 7. Write about longitude ( φ ) and altitude (α). The longitude of a place is the angle between a fixed reference meridian called the prime of first meridian and the meridian of the place. The prime meridian universally adopted is that of Greenwich. Te longitude of any place varies between 0° and 180°, and is reckoned as Φ° east or west of Greenwich. The altitude of celestial or heavenly body (i.e, the sun or a star) is its angular distance above the horizon, measured on the vertical circle passing through the body. 8. Write a short note on: Co-altitude or Zenith Distance (z) and azimuth (A). It is the angular distance of heavenly body from the zenith. It is the complement or the altitude, i.e z = (90° - α). The azimuth of a heavenly body is the angle between the observer’s meridian and the vertical circle passing through the body. 9. Write a short note on: Declination ( δ ) and Co-declination or Polar Distance (p). The declination of a celestial body is angular distance from the plane of the equator, measured along the star’s meridian generally called the declination circle, (i.e., great circle passing through the heavenly body and the celestial pole). Declination varies from 0° to 90°, and is marked + or – according as the body is north or south of the equator.
- Spring '20
- Celestial coordinate system, great circle, base line