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The Reasons for Seasons

2010

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Day Night Cycle

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Rotation vs. Revolution

• Rotation is the spin of an object about its axis.– The Earth rotates once a day (once

every 24 hours).

• Revolution is the orbit of one object around another.– The Earth revolves around the Sun

every 365.26 days.

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Rotation versus Revolution

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Tilt of the Earth’s Axis

• The axis around which the Earth rotates is tilted by 23.5 degrees with respect to the ecliptic.

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Tilt of the Earth’s Axis

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Earth on June 21

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Standing on the North Pole

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Midnight Sun

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Seasons Misconception

• Many people carry the misconception that the seasons are due to the distance of the Earth from the Sun. However, consider the following facts:– The Earth’s orbit around the Sun is nearly a

perfect circle. The Earth is slightly closer to the Sun in January and farther from the Sun in July.

• Perihelion (closest to the Sun) is around January 3 when Earth is about 91,405,436 miles from the Sun.

• Aphelion (farthest from the Sun) is around July 4 when Earth is about 94,511,989 miles from the Sun.

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Seasons Misconception

– While it is winter in the Northern hemisphere it is summer in the Southern hemisphere. If the seasons were due to our distance from the Sun both hemispheres would have the same seasons at the same time.

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The Seasons

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The Seasons

• In fact, the seasons are due to the tilt of the Earth’s axis. Consider what happens on June 21 when the northern hemisphere of the Earth is tilted toward the Sun:

• The sunlight strikes the ground more vertically than in December. The light is spread out over less ground and heats the ground better.

• The Sun is above the horizon for a longer period of time.

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The Seasons

• Seasons Animation• Retrograde

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The Length of the Day

• A day is defined as the time that it takes the Earth to rotate on its axis.

• However, there is more than one way to define a day:– A sidereal day is the time that it takes

for the Earth to rotate with respect to the distant stars.

– A solar day is the time that it takes to rotate with respect to the Sun.

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The Length of the Day

• A solar day is slightly longer than a sidereal day.– A sidereal day is 23h 56m 4.091s.

• We set our watches according to the solar day.

• Astronomers use sidereal time because we are mostly interested in distant celestial objects.

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A.M. and P.M.

• At midday, the Sun is on your meridian.– This occurs close to, or at, noon.– A.M. comes from ante meridiem

(before midday)– P.M. comes from post meridiem (after

midday)

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Day Night Cycle

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Time Zones

• Both the mean solar time and the apparent solar time differ with longitude.– Imagine starting in Charlottesville at exactly

noon.– As you travel to the west, the Sun will

appear further east in the sky (i.e. lower and further from the meridian).

– Even if you travel only a few miles west, the Sun moves off the meridian.

– Each city would have its own time.

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Time Zones

• With the advent of rapid travel by trains in the 19th century, it became necessary to standardize the time for all cities within a certain region.

• In November 1883, the railroad companies divided the United States into four time zones.– Everyone in a time zone set their

clocks to the same standard time.

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Time Zones

• In 1884, an international conference was held in Washington D.C. by 26 countries.– The world was divided into 24 times zones,

with each zone being roughly 15 degrees wide in longitude.

– Time zones have been modified for political, social and economic reasons.

– Since there are 24 hours in a day, and 360/15=24, the time in each zone differs from the time in adjacent zones by one hour.

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International Date Line

– Standard time gets earlier as you travel to the west.

– The International Date Line line was established in the middle of the Pacific Ocean.

– As you go from east to west, you gain a day as you cross the line.

– As you go from west to east, you lose a day as you cross the line.

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Time Zones

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Daylight Saving Time

• During the late spring, summer, and early fall, we set out clocks ahead to have an extra hour of daylight at the end of the day.– This change in time is called Daylight Saving

Time.– The idea of changing our clocks was first used in the

United States during World War I to conserve energy.

• Since 1986, the United States has set our clocks one hour ahead on the first Sunday in April and one hour behind on the last Sunday in October.– Spring ahead, fall back.

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Length of a Year

• It takes Earth one year to orbit the Sun– A sidereal year is the length of time it

takes for the Earth to return to the same position with respect to the stars. It is 365d 6h 9m 9.5s

– A tropical year is the length of time it takes the Sun to go from one vernal equinox to another. It is 365d 5h 48m 45.51s

– The difference is due to precession.