Reasons for Seasons Question: TRUE OR FALSE?
Earth is closer to the Sun in summer and farther from the Sun in winter.
Question: TRUE OR FALSE?
Earth is closer to the Sun in summer and farther from the Sun in winter.
Hint: When it is summer in the U.S., it is winter in Australia.
Question: TRUE OR FALSE Earth is closer to the Sun in summer and farther from the Sun in winter.
Seasons are opposite in the N and S hemispheres, so orbital distance cannot be the reason.
What causes the seasons?
Tilt of the Earth’s rotation axis .
Earth’s rotation axis always points to Celestial North Pole, tilted 23.5 degs from the axis of the Earth’s orbital axis. (Drawing not to scale!)
June 21
March 21
Dec 21
What causes the seasons?
Tilt of the Earth’s rotation axis .
Earth’s rotation axis always points to Celestial North Pole, tilted 23.5 degs from the axis of the Earth’s orbital axis. (Drawing not to scale!)
June 21:
Summer solstice
March 21: Spring equinox
Dec 21:
Winter solstice (Labels only for N. Hemisphere)
What causes the seasons? What causes the seasons?
Summer solstice What causes the seasons?
Winter Solstice
Summary: The Reason for Seasons
• Earth’s axis points in the same direction all year round (to Celestial North Pole), so its orientation relative to the Sun changes as Earth orbits the Sun.
Why doesn’t distance matter?
• The Earth’s orbit is nearly an exact circle.
Only small (3%) variation of the Earth-Sun distance over the orbit.
(In fact, Earth is closer to the Sun in the Southern Hemisphere winter.)
• The small variation in the Earth’s distance is
overwhelmed by effects of axis tilt.Note: Some planets have greater distance variation in their orbit that DOES affect their seasons (Mars, Pluto).
Progression of the seasons
summer solstice: maximum tilt towards Sun for N. Hemisphere.
• June 21 (“first day of summer”)
winter solstice: maximum tilt away from Sun for N. Hemisphere.
•Dec 21 (“first day of winter”)
spring equinox, fall equinox:
Sun shines equally on both hemispheres
We can recognize solstices and equinoxes by Sun’s path across sky:
Summer solstice:
• highest path (longest day)
• rise and set at most extreme north of due east.
Winter solstice:
• lowest path (shortest day)
• rise and set at most extreme south of due east.
Equinoxes:
• Sun rises precisely due east and sets precisely due west.
• Day is precisely 12 hours long.
We can recognize solstices and equinoxes by Sun’s path across sky:
Summer solstice:
• Sun follows the highest path in the sky (longest day)
• Rise and set at most extreme north of due east.
We can recognize solstices and equinoxes by Sun’s path across sky:
Winter solstice:
• Sun follows the lowest path in the sky (shortest day)
• Rise and set at most extreme south of due east.
We can recognize solstices and equinoxes by Sun’s path across sky:
Equinoxes:
• Sun rises precisely due east and sets precisely due west.
• Person at the Equator will see Sun passing directly overhead.
Seasonal changes are more extreme at high latitudes
Path of the (circumpolar) Sun on the summer solstice at the Arctic Circle (66 degrees N latitude)
Does the orientation of Earth’s axis change with time?
• YES. It actually precesses over about 26,000 yrs.
Does the orientation of Earth’s axis change with time?
• YES. It actually precesses over about 26,000 yrs.
• The rotation axis tilt remains 23.5 degs (so the seasonal pattern is not affected), but the axis orientation changes very slowly over time.
• Polaris wonʼt always be the North Star !
• Constellations in which the Sun is located shift 360 degrees around the Zodiac over this 26,000 year period.
• 2000 yrs ago the spring equinoxes occurred with the Sun in Aries.
• Now they occur in Pisces.
• In another 2000 years, they will be in Aquarius (“Age of Aquarius”).
The Moon: Phases and Eclipses
Full moon viewed from the Space Shuttle: which direction is the Sun?
Phases of the Moon: 29.5-day cycle
It is safe to try this at home!
Earth and Moon viewed from the Galileo spacecraft on its way to Jupiter
A quarter Earth seen from the Moon (Apollo 8, December 1968)
Eclipses
What causes eclipses?
• The Earth and Moon cast shadows.
• When either passes through the other’s shadow, we can see an eclipse on the Earth.
When can lunar eclipses occur?
• Lunar eclipses can occur only at full moon (when the Earth is between the Moon and the Sun.)
• Lunar eclipses can be penumbral, partial, or total.
Total lunar eclipse
Why is the Moon red, not black?
When can solar eclipses occur?
• Solar eclipses can occur only at new moon (when the Moon is between the Sun & the Earth).
• Solar eclipses can be partial, total, or annular.
• Solar eclipses only occur over part of the Earth.
La Paz, Mexico: July 1990
Africa: June 2001
Total solar eclipse
One of the most spectacular natural wonders to witness
1991: Baja, Mexico (also seen in Hawaii)
Why do not we have an eclipse at every new and full moon?
– The Moon’s orbit is tilted 5° to ecliptic plane…
– So we have about two eclipse seasons each year, with a lunar eclipse at new moon and solar eclipse at full moon.
Two conditions must be met to have an eclipse:
1. It must be full moon (for a lunar eclipse) or new moon (for a solar eclipse).
AND
2. The Moon must be at or near one of the two points in its orbit where it crosses the ecliptic plane (its nodes).
