Back To CourseBasics of Astronomy
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One little factor sometimes makes all the difference. Planet Earth should be lucky that its axis of rotation is tipped 23.4 degrees. This one 'little' factor is what allows those of you who enjoy the four seasons to, well, enjoy the four seasons!
Had the Earth rotated fully upright in its orbit, had its axis of rotation been perpendicular to the plane of its orbit, Earth would not have different seasons. Those of you who love summer 24/7/365 may not care much for this lesson, but everyone else should listen up as we go over the cycle of the seasons on Earth.
Since Earth's axis of rotation is inclined 23.4 degrees, the Sun will be in the northern sky in the spring and summer and the southern sky during the fall and winter. Essentially, this is what causes this cycle to occur.
To help you understand why, we need to first imagine that an imaginary sphere, called the celestial sphere, surrounds the Earth. On this celestial sphere are drawn two circles. One is called the celestial equator, which is just the projection of Earth's equator on this celestial sphere. The other is called the ecliptic, which can be thought of as the projection of Earth's orbit on the sky and the apparent path of the Sun on the celestial sphere.
As you can tell by the image below, the celestial equator and ecliptic are tipped to one another by 23.4 degrees.
As the Sun moves northward and crosses the celestial equator, it will pass the point called the vernal (or March) equinox, the time when spring begins in the Northern Hemisphere and fall begins in the Southern Hemisphere. This happens on about March 21 of every year. 'Vernal' comes from the word for 'spring,' and 'equinox' comes from the word 'equal,' implying that each day will have the same amount of daylight and darkness everywhere on Earth.
After the vernal equinox, the Sun will be at its most northerly point around June 21 at the summer (or northern) solstice, the beginning of summer in the Northern Hemisphere and winter in the Southern Hemisphere. 'Solstice' comes from the Latin for 'solar standstill' because the Sun will stop moving northward at this point and will move southward thereafter. During the summer solstice, Earth's Northern Hemisphere is tilted toward the Sun, resulting in lots of daylight and shorter nights.
Summer isn't hot only because the daylight hours are extended, but also because of the way the sunlight hits the hemisphere during its summer. At such a time, the sunlight hits the ground at a near perpendicular angle, and this heats the ground very efficiently. In the opposite hemisphere, where it's winter, the sunlight hits the ground at an angle, and this doesn't warm the ground very well.
Look at the images below to understand why. When the sun strikes the ground virtually head on, it concentrates all of its heat-giving energy on a smaller area of land. This means that smaller area of land receives lots of energy and heats up really well. During the winter, the same piece of land is struck with light at an angle. This causes the sunlight to spread out over a larger area, diminishing how much energy any one section of land can receive, and so the ground doesn't heat up as much.
This would be like taking a flashlight and pointing it almost directly downwards at a table vs. pointing it at an angle to simulate the winter Sun. You can see how in the latter case, the flashlight lights up a larger surface area and thus its limited output of energy is spread out over a larger area.
So, after this summer solstice, the Sun will move southward and cross the celestial equator once again at a point called the autumnal (or September) equinox, which is the time when autumn begins in the Northern Hemisphere and spring in the Southern Hemisphere, roughly around September 22nd of every year.
As it keeps going southward, the Sun will reach its southernmost point around December 21, at the winter (or southern) solstice, the time winter begins in the Northern Hemisphere and summer begins in the Southern Hemisphere.
During the winter solstice, Earth's Northern Hemisphere is tipped away from the Sun. Thus, the Northern Hemisphere has shorter daylight hours, longer nights, and cooler weather compared to the southern one for all the reasons we described before.
It used to be believed that the variations in the seasons are actually caused by the distances between the Earth and the Sun. Since Earth's orbit isn't a perfect circle (it's slightly elliptical), it was believed this greatly influenced the seasons. It would make perfect sense then that the Earth will be closest to the Sun during summer and farthest away during winter. It makes sense to me.
The point in Earth's orbit where it's closest to the Sun is the perihelion, and the point in Earth's orbit where it's farthest away from the Sun is called the aphelion. Remember, 'a'phelion is farthest 'a'way from the Sun.
The perihelion occurs around January 3rd, and aphelion occurs around July 5th.
Umm, I think there's a problem with my theory. See what's going on? In the Northern Hemisphere, the Earth is closer to the Sun in wintertime than during summertime! This means that for this theory to hold up, the seasons would have to be the same in both hemispheres, and they're clearly not.
The end point here is that the Earth-Sun distance varies by only about 3% during the year, and this small variation has little influence on the cycle of the seasons.
Let's review the important points of the cycle of the seasons on Earth.
The vernal (or March) equinox is the time when spring begins in the Northern Hemisphere and fall in the Southern Hemisphere. This happens on about March 21 of every year.
The summer (or northern) solstice occurs around June 21, and it marks the beginning of summer in the Northern Hemisphere and winter in the Southern Hemisphere.
Around September 22nd, the Sun passes the autumnal (or September) equinox, which is the time when autumn begins in the Northern Hemisphere and spring in the Southern Hemisphere.
At approximately December 21st, the Sun reaches its southernmost point at the winter (or southern) solstice, the time winter begins in the Northern Hemisphere and summer begins in the Southern Hemisphere.
Remember, it's Earth's tilt, the amount of daylight, and the way that the light strikes the ground that determines the cycle of the seasons and the temperature of the season.
The point in Earth's orbit where it's closest to the Sun, the perihelion, and the point in Earth's orbit where it's farthest away from the Sun, called the aphelion, vary so little throughout the year that they have almost no influence on the cycle of the seasons.
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Back To CourseBasics of Astronomy
28 chapters | 325 lessons