View
9
Download
3
Category
Tags:
Preview:
DESCRIPTION
Citation preview
04/08/23 The Seasons 1
The Reasons for Seasons
2010
04/08/23 The Seasons 2
Day Night Cycle
04/08/23 The Seasons 3
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.
04/08/23 The Seasons 4
Rotation versus Revolution
04/08/23 The Seasons 7
Tilt of the Earth’s Axis
• The axis around which the Earth rotates is tilted by 23.5 degrees with respect to the ecliptic.
04/08/23 The Seasons 8
04/08/23 The Seasons 11
Tilt of the Earth’s Axis
04/08/23 The Seasons 18
Earth on June 21
04/08/23 The Seasons 19
Standing on the North Pole
04/08/23 The Seasons 20
Midnight Sun
04/08/23 The Seasons 27
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.
04/08/23 The Seasons 28
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.
04/08/23 The Seasons 30
04/08/23 The Seasons 31
The Seasons
04/08/23 The Seasons 32
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.
04/08/23 The Seasons 33
The Seasons
• Seasons Animation• Retrograde
04/08/23 The Seasons 37
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.
04/08/23 The Seasons 38
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.
04/08/23 The Seasons 41
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)
04/08/23 The Seasons 43
Day Night Cycle
04/08/23 The Seasons 52
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.
04/08/23 The Seasons 53
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.
04/08/23 The Seasons 54
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.
04/08/23 The Seasons 55
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.
04/08/23 The Seasons 56
Time Zones
04/08/23 The Seasons 57
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.
04/08/23 The Seasons 62
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.
Recommended