The Ecliptic - Essay Example

The Ecliptic The Ecliptic The path of the Suns motion as seen from Earth on the celestial sphere is known as the ecliptic. The is called ecliptic in view of the occurrence of eclipses when the full Moon approaches to the path of the Sun. Source: http://www-istp.gsfc.nasa.gov/stargaze/Secliptc.htm Ecliptic and Equator Rotational axis of the Earth is not perpendicular to the orbital plane and also ecliptic plane as well as equatorial planes are not parallel but form an angle of 23°26; it is called axial tilt or is also known as obliquity of the ecliptic. Obliquity or axial tilt, in astronomy, is the angle forming between rotational axis and a line perpendicular to plane of orbit. Celestial Sphere It can be imagined that the stars as point of light on a sphere rotates about the Earth and projecting the Earths equator and poles onto this imaginary sphere gives a framework for celestial phenomena. Viewing the directions for formal measurement from the Earth are called ascension and declination something like longitude and latitude as understood on the surface of the Earth. Axis of Tilt and Seasons Seasons on the Earth are formed due to tilt of the Earths axis with respect to the ecliptic. The tilt has a special significance in the evolution of life on the Earth. Tilt of axis causes seasons; in summer of northern hemisphere, Sun rays falling on the Earth’s plane at 90 degrees cause evaporation of surface water in the sea forming the clouds in due course of time and that brings rains over many parts. Same is true for southern hemisphere. Seasons are opposite for northern and southern hemisphere. Thus, it is the tilt of axis that causes rains on the Earth. Source: http://hyperphysics.phy-astr.gsu.edu/hbase/eclip.html#c2 The Rotation of the Celestial Sphere It is to be kept in mind that ecliptic does not represent the daily path of the Sun but it represents an Annual Drift of the Sun. The rotation of the celestial sphere is responsible for the Suns daily motion across the sky. The Suns position on the ecliptic will tell us how high or how low Sun will carry in the sky in a given day, indicating whether we are passing through summer or winter seasons. The Sun’s travelling along the ecliptic and its path on two dates, June 1 (in Taurus) and December 1 (in Scorpius) can be seen in Figure 1 as shown hereunder. The path of the Sun and the wobble of the ecliptic at 6-hour intervals can be seen in the figures 2, 3 and 4 as the celestial sphere rotates. Source: Pearson Prentice Hall Equinox The two points where ecliptic plane and the celestial plane intersect each other on the celestial sphere are known as Equinox. When Sun crosses the celestial equator from south to north, the first point is called the vernal equinox. This occurs on March 21 indicating the beginning of spring in the Northern Hemisphere. The same phenomenon repeats on September 23 when the Sun again crosses the celestial equator, from north to south, marking the beginning of autumn in the Northern Hemisphere. On the point of equinoxes, the Earth will have equal daylight and dark of 12 hours. Precession of the Equinoxes The equinoxes are not stationary points on the celestial map but move westward along the ecliptic and passes through all the constellations of the zodiac in the period of 26,000 years. This is known as precession of the equinoxes. The precession is because of the gravitational pull of the Moon and Sun on the equatorial bulge of the Earth. It should be noted that ecliptic is not affected by this motion. Celestial Measurement In order to locate a particular object in the sky, it is necessary to have a precise measurement of time and precise description of the Earths motion around the Sun. Thus, celestial measurement essentially denotes the measurement with respect to the celestial sphere so that it is easy to locate the astronomical objects for observation. Declination and Right Ascension Right Ascension, also called celestial longitude, is measured in the direction of the Earths rotation. It makes one complete circle in 24 hours and measured in hours and minutes. On the other hand Declination is measured as an angle in reference to the celestial equator. They are used to denote the location of the stars. Source: http://hyperphysics.phy-astr.gsu.edu/hbase/eclip.html#c5 Using this understanding for measuring the location of the star Betelgeuse in the constellation Orion, it can be said to having the right ascension of 5 hours 52 minutes and the declination of 7 degrees 24 minutes. Ecliptic and Eclipses The Moons orbit intersects the ecliptic at narrow angle of about 5 degrees; it means that Moon, too, on the celestial sphere follows a path through zodiac. Moon moves north and south of the ecliptic equally. The Sun is said to be eclipsed when shadow of the Moon hits the Earth similarly the Moon is said to be eclipsed when shadow of the Earth falls on the Earth. These phenomena can happen only when Earth, Moon and Sun fall on the straight line and in that event Moon is also said to be in plane of ecliptic as well. Moon goes around the Earth and completes one revolution in a month and during that time it crosses the ecliptic twice. At that time Sun may be at any place along the ecliptic. When it is not on Earth-Moon line, there is no possibility of eclipse; however, occasionally Sun arrives at the same spot on the celestial sphere in which Moon covers the Sun causing an eclipse of the Sun. Same phenomenon is repeated when the Earth falls between Moon and Sun causing an eclipse of the Moon. Synodic Periods The time required for any planet including artificial Earth satellite to return to the same position relative to the Sun when observed from the Earth. For example, the Moons synodic period will be the time required for the recurrences of the same state such as full Moon to full Moon. The synodic period of a planet can be described as time required for the Earth to overtake it as both move around the Sun. The synodic period of the Earth is shorter if seen by an observer at the planet farther from the Sun in comparison to the Earth. Against this, the synodic period of the Earth is longer if seen by an observer at the planet closer to the Sun than the Earth. Sidereal Period This is the time required for any celestial body within the solar system to make one full revolution in reference to the fixed stars, when observed from some stationary point outside the system. If the synodic period is known, the sidereal period of a planet can be determined. Conclusion The understanding of ecliptic explains us many celestial events including the Sun and Moon eclipses. Knowing about the apparent motion of the Sun and other planets across the celestial sphere also indicates that our solar system is flat and disk-shaped. References: 1. Chaisson, Eric, McMillan, Steve. Astronomy Today: The Solar System, Volume I, 7th edition, Amazon 2. The Path of the Sun, the Ecliptic 16 April 2011 http://www-istp.gsfc.nasa.gov/stargaze/Secliptc.htm 3. Ecliptic Plane 16 April 2011 http://hyperphysics.phy-astr.gsu.edu/hbase/eclip.html#c5 4. Becker, Gary A. (2000) Equatorial Coordinates 16 April 2011 http://www.astronomy.org/astronomy-survival/coord.html 5. Synodic period 16 April 2011 http://www.britannica.com/EBchecked/topic/578497/synodic-period 6. Understanding Stars 16 April 2011 http://pachamamatrust.org/f2/K/navigation/Nb_understanding_stars_KN.htm 7. Sidereal period 16 April 2011 http://www.britannica.com/EBchecked/topic/543027/sidereal-period Read More