LECTURE 3: SEASONS, LUNAR PHASES, AND ECLIPSES

Key Questions:






SEASONS

Summer is hot, winter is cold.
An obvious explanation: Earth is closer to Sun in summer, further in winter.
This explanation makes predictions contrary to actual experience, so it cannot be correct.

Real explanation:

Seasonal changes are caused by tilt of Earth's rotation axis.
Equator is tipped by 23.5 degrees relative to Earth's orbit around the Sun.

Tilt of ecliptic (and hence existence of seasons) is caused by tilt of Earth's rotation axis, which stays fixed as Earth orbits Sun.
As a result, summer and winter are opposite in northern and southern hemisphere.
In north:

In south, June 21 is Winter Solstice and Dec. 21 is Summer Solstice.






LUNAR PHASES

Phases arise as Sun illuminates different parts of Moon as seen from Earth.
Depends on Sun-Moon-Earth angle.








PERIOD OF THE MOON

Synodic period: Time required to return to same position relative to Sun (e.g., full Moon to full Moon).
Sidereal period: Time required to return to same position relative to stars.

The Moon's synodic period, 29.5 days, is called a month.
It differs from the Moon's sidereal period, 27.3 days, because the Sun moves relative to the stars (as Earth orbits the Sun).
There are 12.38 synodic periods (a.k.a. months) per year, 13.38 sidereal periods (1 more).
Can use Moon for calendar, but lunar calendar complicated because 12.38 is not a whole number.






ECLIPSES

What happens when Earth gets exactly in between Sun and Moon?
Earth's shadow falls on Moon, Moon goes dark.
Lunar eclipse. (Eclipse of Moon by Earth.)

What happens when Moon gets exactly in between Sun and Earth?
Moon's shadow falls on Earth, Sun blocked.
Solar eclipse. (Eclipse of Sun by Moon.)

Paths of Sun and Moon slightly inclined (by 5 degrees).
Eclipses can only occur when path of Moon crosses the path of Sun, a.k.a. the ecliptic.
Moon crosses ecliptic twice a month, but eclipse only occurs if Sun, Earth, and Moon are properly aligned when this happens.






LUNAR ECLIPSES








SOLAR ECLIPSES








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Updated: 2005 March 27[dhw]