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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)