THE SEASONS

When do the seasons start and end?

When it comes to climate and weather, the seasons start at different times of year depending on where one is on Earth. Astronomically speaking, though, the first day of spring happens on the vernal equinox; the first day of summer happens on the summer solstice; the first day of fall happens on the autumnal equinox; and the first day of winter happens on the winter solstice.

When it comes to official weather statistics, the seasons are considered to be as follows: winter is December through February; spring is March through May; sum-

How does weather affect Earth's rotation?

Imagine the water, clouds, and other gases lying on top of the Earth's crust as a big soupy mass that can shift around as the planet rotates on its axis and is tugged on by the Moon, Sun, and other planets. The oceans and atmosphere slosh around due to tidal action, bulging a bit on one side or the other, and this can hamper or speed up the planet's motion. In comparison to the total weight of our planet, the liquids and gases are fairly light, but the inertia they experience does, in fact, change Earth's speed. The amount of change is not noticeable to us: a few thousandths of a second each year. Over millions of years, however, this has a cumulative effect.

mer is June through August; and fall is September through November. So, if you hear a report, for example, that "last summer was the hottest on record," that means June 1 through August 31, and not June 21 through September 21, which is how it is marked on your typical calendar.

What is the ecliptic plane?

The ecliptic plane is the plane of Earth's orbit around the Sun. Ancient astronomers were able to trace the ecliptic as a line across the sky, even though they did not know Earth actually orbited the Sun. They merely followed the position of the Sun compared to the position of the stars in the sky, figured out (despite the Sun drowning out the light of the other stars) where the Sun was every day, and noticed that every 365 days or so the positions would overlap and start going over the same locations again. That line marked a loop around the celestial sphere. Astronomers marked the line using twelve zodiac constellations positioned near and through the loop.

What is the difference between the ecliptic plane and Earth's equatorial plane?

The equatorial plane is the plane of Earth's equator extended indefinitely out into space. It turns out that Earth's rotation around its axis is not lined up with the ecliptic plane. Instead, Earth is tilted about 23.5 degrees. This tilt is the main cause of the seasons on Earth.

How does the motion of Earth around the Sun cause the seasons to occur?

Some people mistakenly think that the seasons are caused by Earth being farther from the Sun in winter and closer to the Sun in summer. This is incorrect; Earth's elliptical orbit is close enough to a perfect circle that distance is not the reason. In fact, Earth is closest to the Sun in early January and farthest in early July, which is exactly the opposite of our summer and winter seasons. The reason for the seasons has to do with the angle at which sunlight strikes any particular place on Earth at any given time of year. The angle changes throughout the year because the tilt of Earth's axis differs from the ecliptic. Since the Earth is tilted 23.5 degrees, the Sun's rays hit the northern and southern hemispheres unequally. When the Sun's rays hit one hemisphere directly, the other hemisphere receives diffused rays. The hemisphere that receives the direct rays of the Sun experiences summer; the hemisphere that receives the diffused rays experiences winter. Thus, when it is summer in North America, it is winter in most of South America, and vice versa.

 
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