PHY115: Professional Skills in Physics and Astronomy

Positional Astronomy: Session 2

The relationship between the Earth's daily rotation, and its orbital plane. Source: wikimedia commons.
The Ecliptic

To fully understand the consequences of the Sun's motion around the celestial sphere, we need to know how to predict it's position on the sphere at any time of year. The path that the Sun follows round the celestial sphere is known as the ecliptic. The Sun always lies in the plane of the Earth's orbit, and so the intersection of this plane with the celestial sphere defines the ecliptic.

The Earth's spin axis is inclined with respect to its orbit. This misalignment of spin axis and orbit is called the axial tilt, or obliquity, ε. The amount of obliquity is currently 23° 27'. Since the poles of the celestial sphere are aligned with the spin axis of the Earth, the plane of Earth's orbit traces an inclined circle on the celestial sphere; this is the ecliptic. The Sun lies on the ecliptic at all times, and traces a path around it over the course of one year (365.242 mean solar days), as shown below.
The ecliptic.
The ecliptic crosses the celestial equator at two points, located in the constellations of Aries and Libra. As a result, twice a year the Sun has a declination of zero. The first point of aries, , is defined as the point where the Sun crosses the celestial equator from south to north. This occurs roughly around March 20th, the spring equinox, sometimes called the vernal equinox. The second date when the Sun has declination of zero is when it crosses the celestial equator from north to south, know as the autumnal equinox, which occurs around September 22nd. Since the Sun has a declination of zero on the dates of equinox, those dates see 12 hours of daylight and 12 hours of night almost everywhere on the Earth (use the coordinate system comparison applet at University of Nebraska to see this for yourself). One more point before moving on. In the last session, we discussed the celestial co-ordinate system, and said that we had to make a choice for the location of zero right ascension. A natural choice for this point is where the Sun crosses the celestial equator, which is why the first point of Aries defines zero right ascension.

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