The Seasons

Question 1

How do seasons occur?

Answer 1

Seasons result from variations in the Sun’s altitude above the horizon, the Sun’s declination (latitude of the subsolar point), and daylength during the year. These in turn are created by several physical factors that operate in concert: Earth’s revolution in orbit around the Sun, its daily rotation on its axis, its tilted axis, the unchanging orientation of its axis, and its sphericity (summarized in Table 2.1). Of course, the essential ingredient is having a single source of radiant energy—the Sun.

Question 2

What is earths Revolution?

Answer 2

Orbit around the Sun; requires 365.24 days to complete at 107 280 km·h-1

This speed, together with Earth’s distance from the Sun, determines the time required for one revolution around the Sun and, therefore, the length of the year and duration of the seasons. Earth completes its annual revolution in 365.2422 days. This number is based on a tropical year, measured from equinox to equinox, or the elapsed time between two crossings of the equator by the Sun

Question 3

Describe Earths Rotation

Answer 3

Earth’s rotation, or turning on its axis, is a complex motion that averages slightly less than 24 hours in duration. Rotation determines daylength, creates the apparent deflection of winds and ocean currents, and produces the twice-daily rise and fall of the ocean tides in relation to the gravitational pull of the Sun and the Moon

Question 4

Describe earths tilt.

Answer 4

Alignment of axis at about 23.5° angle from perpendicular to the plane of the ecliptic (the plane of Earth’s orbit)

Question 5

Axial parallelism

Answer 5

Unchanging (fixed) axial alignment, with Polaris directly overhead at the North Pole throughout the year

Question 6

Sphericity

Answer 6

Oblate spheroidal shape lit by Sun’s parallel rays; the geoid

Question 7

What is the average Earth - Sun distance?

Answer 7

Earth–Sun distance averages 150 million km

Question 8

Significance of Earths Axis.

Answer 8

When viewed from above the North Pole, Earth rotates counterclockwise about its axis, an imaginary line extending through the planet from the geographic North Pole to the South Pole. Viewed from above the equator, Earth rotates west to east, or eastward. This eastward rotation creates the Sun’s apparent westward daily journey from sunrise in the east to sunset in the west. Of course, the Sun actually remains in a fixed position in the centre of our Solar System.

Question 9

What is the circle of illumination?

Answer 9

Earth’s rotation produces the diurnal (daily) pattern of day and night. The dividing line between day and night is the circle of illumination

Question 10

What happens as a result of the circle of illumination intersecting with the the equator?

Answer 10

Because this day-night dividing circle of illumination intersects the equator (and because both are great circles, and any two great circles on a sphere bisect one another), daylength at the equator is always evenly divided—12 hours of the day and 12 hours of the night. All other latitudes experience uneven day length through the seasons, except for 2 days a year, on the equinoxes.

Question 11

What is Mean Solar time?

Answer 11

The length of a true day varies slightly from 24 hours throughout the year. However, by international agreement, a day is defined as exactly 24 hours, or 86 400 seconds, an average called mean solar time

Question 12

What is the Plane of the ecliptic?

Answer 12

To understand Earth’s axial tilt, imagine a plane (a flat surface) that intersects Earth’s elliptical orbit about the Sun, with half of the Sun and Earth above the plane and half below. Such a plane, touching all points of Earth’s orbit, is the plane of the ecliptic. Earth’s tilted axis remains fixed relative to this plane as Earth revolves around the Sun

Question 13

What is Axial parallelism?

Answer 13

Throughout our annual journey around the Sun, Earth’s axis maintains the same alignment relative to the plane of the ecliptic and to Polaris and other stars. You can see this consistent alignment in Geosystems in Action, Figure GIA 2.2 (page 57). If we compared the axis in different months, it would always appear parallel to itself, a condition is known as axial parallelism.

Question 14

What are the Solstices?

Answer 14

Daylength is the most obvious way of sensing changes in season at latitudes away from the equator. The extremes of daylength occur in December and June. The times around December 21 and June 21 are solstices. Strictly speaking, the solstices are specific points in time at which the Sun’s declination is at its position farthest north at the Tropic of Cancer, or south at the Tropic of Capricorn.

Question 15

What are the “Tropics”?

Answer 15

“Tropic” is from tropicus, meaning a turn or change, so a tropic latitude is where the Sun’s declination appears to stand still briefly (Sun stance, or sol stice) and then “turn” and head toward the other tropic.

Question 16

When dose the suns declination change at a faster rate?

Answer 16

You may have noticed that these daily variations become more pronounced in spring and autumn when the Sun’s declination changes at a faster rate

Question 17

What occurs during the December Solstice?

Answer 17

On December 21 or 22, at the moment of the December solstice, or Northern Hemisphere winter solstice (“winter sun stance”), the circle of illumination excludes the North Pole region from sunlight but includes the South Pole region.

The subsolar point is about 23.5° S latitude, the Tropic of Capricorn parallel. The Northern Hemisphere has tilted away from these more direct rays of sunlight—our northern winter—thereby creating a lower angle for the incoming solar rays and thus a more diffuse pattern of insolation

For locations between about 66.5° N and 90° N (the North Pole), the Sun remains below the horizon the entire day. The parallel at about 66.5° N marks the Arctic Circle; this is the southernmost parallel (in the Northern Hemisphere) that experiences a 24-hour period of darkness. During this period, twilight and dawn provide some lighting for more than a month at the beginning and end of the Arctic night.

Question 18

What happens during the March equinox?

Answer 18

The moment of the March equinox, or vernal equinox in the Northern Hemisphere, occurs on March 20 or 21. At that time, the circle of illumination passes through both poles, so that all locations on Earth experience a 12-hour day and a 12-hour night. People living around 40° N latitude (New York, Denver) have gained 3 hours of daylight since the December solstice. At the North Pole, the Sun peeks above the horizon for the first time since the previous September; at the South Pole, the Sun is setting—a dramatic 3-day “moment” for the people working the Amundsen–Scott South Pole Station.

Question 19

What occurs during the June Solstice?

Answer 19

From March, the seasons move on to June 20 or 21, the moment of the June solstice, or summer solstice in the Northern Hemisphere. The subsolar point migrates from the equator to 23.5° N latitude, the Tropic of Cancer. Because the circle of illumination now includes the North Polar region, everything north of the Arctic Circle receives 24 hours of daylight—the Midnight Sun. In contrast, the region from the Antarctic Circle to the South Pole (66.5°–90° S latitude) is in darkness. Those working in Antarctica call the June solstice Midwinter’s Day.

Question 20

What occurs during the September equinox?

Answer 20

September 22 or 23 is the time of the September equinox, or autumnal equinox in the Northern Hemisphere, when Earth’s orientation is such that the circle of illumination again passes through both poles, so that all parts of the globe experience a 12-hour day and a 12-hour night. The subsolar point returns to the equator, with days growing shorter to the north and longer to the south. Researchers stationed at the South Pole see the disk of the Sun just rising, ending their 6 months of darkness. In the Northern Hemisphere, autumn arrives, a time of many colourful changes in the landscape, whereas in the Southern Hemisphere it is spring.

Question 21

Describe dawn and twilight

Answer 21

Dawn is the period of diffused light that occurs before sunrise. The corresponding evening time after sunset is twilight. During both periods, light is scattered by molecules of atmospheric gases and reflected by dust and moisture in the atmosphere. The duration of both is a function of latitude, because the angle of the Sun’s path above the horizon determines the thickness of the atmosphere through which the Sun’s rays must pass. The illumination may be enhanced by the presence of pollution aerosols and suspended particles from volcanic eruptions or forest and grassland fires.

Question 22

How long do the poles experience twilight and dawn?

Answer 22

The poles experience about 7 weeks of dawn and 7 weeks of twilight, leaving only 2.5 months of “night” during the 6 months when the Sun is completely below the horizon