AST101H1 Study Guide - Quiz Guide: Ecliptic, Tidal Locking, Lunar Phase
DepartmentAstronomy & Astrophysics
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Many people guess that seasons are caused by variations in Earth’s distance from the Sun, but if that were the case, we’d
expect it to be warmest (summer) when Earth is closest to the Sun. As you can see in the rankings, Earth is
actually farthest from the Sun when it is summer in the Northern Hemisphere. We conclude that variations in the Earth-Sun
distance from are not the major cause of our seasons
The fact that the two hemispheres have opposite seasons is fur ther proof that the seasons are not caused by
Ear th’s varying distance from the Sun. Keep in mind that Earth is so small in size (diameter 12,800 km)
compared to its distance from the Sun (about 150 million km) that there is no significant difference in the
distances of the two hemispheres from the Sun; therefore, if the seasons were caused by Earth’s varying
distance from the Sun, the entire Earth would have summer at the same time.
As you’ve already seen in Par ts A and B, seasons on Earth are not affected by the variation in Earth’s orbital
distance over the course of each year. This is probably not too surprising when you realize that Earth always
stays within about 3 percent of its average distance from the Sun. Other factors are much more important in
creating the seasons.
The tilt of Earth 's axis causes dif ferent por tions of the Earth to receive more or less direct sunlight
at different times of year.
Without axis tilt, we would not have seasons on Ear th. Seasons on other planets occur for the
same reason, which is why planets with axis tilts (such as Mars and Saturn) have seasons and planets
with very little axis tilt (such Jupiter or the “upside-down” axis of Venus) do not.
Notice that the figure with the most daylight shows that it is summer in Florida (because the Northern
Hemisphere is tilted toward the Sun). The figure with the second-most daylight is the case with no tilt (tilt = 0°),
for which there would always be the same 12 hours of daylight and 12 hours of darkness each day. The
remaining three cases all show Florida in winter (the Northern Hemisphere is tilted away from the Sun), and the
amount of winter daylight becomes shorter for the more extreme tilts.
The greater the axis tilt, the more extreme the seasonal temperature differences. This is true because on any
par ticular summer day, greater axis tilt means longer daylight and a higher summer Sun, making summers
warmer. Similarly, on any winter day, a greater axis tilt means shorter daylight and a lower winter sun, making
winters colder. That is why this ranking shows the figures in order of declining axis tilt, with the two having the
same axis tilt ranked equal.
The seasonal temperature changes on Mars are quite similar in their pattern to those on Earth because
Mars’s axis tilt is so similar to Earth’s. (Note, however, that while Earth’s varying distance from the Sun plays
essentially no role in our seasons, the same is not true on Mars. Because Mars has a more elliptical orbit and
hence a much greater distance variation than Earth, distance from the Sun plays an impor tant secondary role in
the seasons on Mars.)
Uranus is tipped nearly on its side; its axis tilt of 97.9° is equivalent to 180° – 97.9° = 82.1 °,
but with the “north” pole tipped to below the ecliptic plane. This tilt of close to 90° gives Uranus very
The astronaut shows that the Moon rotates exactly once for each orbit around Earth, which is
an example of what we call synchronous rotation.
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