Lunar Phases, Eclipses and Seasons

Physics 102 Goderya

Astronomy

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

Lunar EclipsesEarth’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.

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.

Total Solar Eclipse

Prominences

Chromosphere and Corona

Conditions for Eclipses Eclipses occur in a cyclic pattern.

Temperature & Sun Rays

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.

Season Markers

The seasons due to inclination of spin axis

AphelionPerihelion

January 3 July 4

September Equinox

March Equinox

DecemberSolstice

JuneSolstice

Sun Rays & Incidence Angle

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.

Light from the sun

Steep incidence → Summer

Shallow incidence → Winter

The Seasons are only caused by a varying angle of incidence of the sun’s rays.

Daylight Hours

Northern summer = southern winter

Northern winter = southern summer

Variation of Daylight hours

Variation of Daylight hours

Common Misconception

Earth’s distance from the sun has only a very minor influence on seasonal temperature

variations.

Sun

Earth in July

Earth in January

Earth’s orbit (eccentricity greatly exaggerated)