Motion of Earth and Moon around the Sun Physics 113 Goderya Chapter(s): 2 and 3 Learning Outcome:

The Sun’s apparent path on the sky is called the Ecliptic. The Sun and Its Motions Due to Earth’s revolution around the sun, the sun appears to move through the zodiacal constellations. The Sun’s apparent path on the sky is called the Ecliptic. Equivalent: The Ecliptic is the projection of Earth’s orbit onto the celestial sphere.

Earth’s rotation is causing the day/night cycle. The Rotation of Earth Earth’s rotation is causing the day/night cycle.

Precession (1) At left, gravity is pulling on a slanted top. => Wobbling around the vertical. The Sun’s gravity is doing the same to Earth. The resulting “wobbling” of Earth’s axis of rotation around the vertical w.r.t. the Ecliptic takes about 26,000 years and is called precession.

Precession (2) As a result of precession, the celestial north pole follows a circular pattern on the sky, once every 26,000 years. It will be closest to Polaris ~ A.D. 2100. There is nothing peculiar about Polaris at all (neither particularly bright nor nearby etc.) ~ 12,000 years from now, it will be close to Vega in the constellation Lyra.

The Cause for Seasons Earth’s axis of rotation is inclined vs. the normal to its orbital plane by 23.5°, which causes the seasons.

Seasons (SLIDESHOW MODE ONLY)

Temperature & Sun Rays

Sun Rays & Incidence Angle The Seasons are only caused by a varying angle of incidence of the sun’s rays. Steep incidence → Summer Light from the sun Shallow incidence → Winter They are not related to Earth’s distance from the sun. In fact, Earth is slightly closer to the sun in (northern-hemisphere) winter than in summer.

Daylight Hours Northern summer = southern winter Northern winter = southern summer

The Phases of the Moon (1) The Moon orbits Earth in a sidereal period of 27.32 days. 27.32 days Moon Earth Fixed direction in space

The Phases of the Moon (2) Fixed direction in space 29.53 days Earth Moon Earth orbits around Sun => Direction toward Sun changes! The Moon’s synodic period (to reach the same position relative to the sun) is 29.53 days (~ 1 month).

The Phases of the Moon (3) From Earth, we see different portions of the Moon’s surface lit by the sun, causing the phases of the Moon.

Lunar Eclipses Earth’s shadow consists of a zone of partial shadow, the Penumbra, and a zone of full shadow, the Umbra. If the moon passes through Earth’s full shadow (Umbra), we see a lunar eclipse. If the entire surface of the moon enters the Umbra, the lunar eclipse is total.

A Total Lunar Eclipse

Why the Red Glow? A total lunar eclipse can last up to 1 hour and 40 min. During a total eclipse, the moon has a faint, red glow, reflecting sun light scattered in Earth’s atmosphere.

Typically, 1 or 2 lunar eclipses per year.

Solar Eclipses The sun appears approx. as large in the sky (same angular diameter ~ 0.50) as the moon.  When the moon passes in front of the sun, the moon can cover the sun completely, causing a total solar eclipse.

Solar Eclipses: 2002-2012 Approximately 1 total solar eclipse per year

Total Solar Eclipse Chromosphere and Corona Prominences

Diamond Ring Effect

Conditions for Eclipses The moon’s orbit is inclined against the ecliptic by ~ 50. A solar eclipse can only occur if the moon passes a node near new moon. A lunar eclipse can only occur if the moon passes a node near full moon.

Conditions for Eclipses Eclipses occur in a cyclic pattern.