LECTURE 3: SEASONS, LUNAR PHASES, AND ECLIPSES
- What causes seasons?
- What causes lunar phases?
- What causes lunar eclipses?
- What causes solar eclipses?
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.
Seasonal changes are caused by tilt of Earth's rotation axis.
- Inclination of ecliptic relative to equator (23.5 degrees) means Sun
rises higher in sky during summer.
- When Sun is higher in sky, get more light per area, so Earth absorbs
- Also, day is longer, more time to absorb energy.
- Distance to Sun does vary over year, but this is a much smaller effect.
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 south, June 21 is Winter Solstice and Dec. 21 is Summer Solstice.
- Shortest day: Winter Solstice, Dec. 21.
- Longest day: Summer Solstice, June 21.
- Sunrise-to-Sunset = 12 hours at Vernal Equinox (March 21) and
Autumnal Equinox (Sept. 21).
Phases arise as Sun illuminates different parts of Moon as seen from Earth.
Depends on Sun-Moon-Earth angle.
- Full Moon: Sun and Moon aligned on opposite sides of Earth.
Sun illuminates entire side of Moon that faces Earth.
- New Moon: Sun and Moon aligned on same side of Earth.
Illuminated side of Moon is away from Earth, and Moon is only
up in the day.
- First quarter, third quarter: Sun-Moon-Earth angle is 90 degrees.
Half of Earth-facing side of Moon is illuminated, half is in shadow.
- Note: quarter means quarter of way through cycle, not quarter
- Full cycle occurs every 29.5 days.
- Subtleties: Moon gets larger and smaller, by ~10%, because its
distance from Earth varies slightly.
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
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.
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.
- Earth must be between Sun and Moon. Lunar eclipses occur only
at full Moon.
- When Moon enters Earth's shadow, goes dark for entire side of Earth
that faces it (where it is night).
- Also, Earth bigger than Moon, so required alignment not exact.
- Moon moves through shadow at (360 deg/month) ~ 0.5 deg/hour.
Eclipses last ~ 1-3 hours, depending on alignment.
- Lunar eclipses occur 2-5 times per year, not so rare to see.
- Moon must be between Sun and Earth. Solar eclipses occur only
at new Moon.
- Sun and Moon have almost identical angular size, 0.5 degree.
- Only small patch of Earth sees Sun blocked by Moon's shadow.
- Location of this patch moves as Earth rotates, so eclipse path
traces a line on Earth.
- Solar eclipse occurs a couple of times per year, but only
a small fraction of Earth's surface sees it.
- At one place, total eclipse lasts only a few minutes.
- A solar eclipse is a rare thing to see.
- When Moon is further from Earth, angular size is slightly smaller,
does not completely block Sun.
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Updated: 2005 March 27[dhw]