Chapter 2.2: The Reason for Seasons

precession

The gradual wobble of the axis of a rotating object around a vertical line.

True or false: Earth is only about 3% farther from the Sun at its farthest point (which is in July) than at its nearest (in January).

True.

True or false: High latitudes have more extreme seasons.

True. For example, Vermont has much longer summer days and much longer winter nights than Florida.

How do we figure out the December solstice?

December solstice, when the Sun rises and sets farthest to the south and the Northern Hemisphere has its shortest hours of daylight and lowest midday Sun.

What is precession caused by?

Precession is caused by gravity's effect on a tilted, rotating object.

What leads to the progression of seasons?

The combination of Earth's rotation and orbit also leads to the progression of the seasons.

Can the dates for solstices and equinoxes vary?

Yes. The exact dates and times of the solstices and equinoxes can vary by up to a couple of days from the dates given above, depending on where we are in the leap year cycle.

How do Mars' seasons work?

Although distance from the Sun plays no role in Earth's seasons, it can affect seasons on planets with more elliptical orbits. For example, Mars has about the same axis tilt as Earth and therefore has similar seasonal patterns, but its much greater variation in distance from the Sun makes these patterns more extreme in its southern hemisphere than in its northern hemisphere

How long does Earth's precession cycle take?

Each cycle of Earth's precession takes about 26,000 years. This gradually changes the direction in which the axis points in space.

True or false: the seasons depend on Earth's distance from the Sun.

False. The seasons do not depend on Earth's distance from the Sun, which varies only slightly throughout the year.

Why do we consider the solstice to be the beginning rather than the midpoint of summer?

First, it was much easier for ancient people to identify the days on which the Sun reached extreme positions in the sky—such as when it reached its highest point on the summer solstice—than other days in between. Second, we usually think of the seasons in terms of weather, and the warmest summer weather tends to come 1 to 2 months after the solstice.

How does precession work?

However, if you spin the top rapidly, it does not fall over so easily. The spinning top stays upright because rotating objects tend to keep spinning around the same rotation axis (a consequence of the law of conservation of angular momentum. This tendency prevents gravity from immediately pulling the spinning top over, since falling over would mean a change in the spin axis from near-vertical to horizontal. Instead, gravity succeeds only in making the axis trace circles of precession. As friction slows the top's spin, the circles of precession get wider and wider, and ultimately the top falls over. If there were no friction to slow its spin, the top would spin and precess forever.

What does precession change?

However, it changes the points in Earth's orbit at which the solstices and equinoxes occur, and therefore changes the constellations that we see at those times.

How are seasons caused by the axis tilt and not by changes in Earth's distance from the Sun?

Many people guess that seasons are caused by variations in Earth's distance from the Sun. But if this were true, the whole Earth would have summer or winter at the same time, and it doesn't: The seasons are opposite in the Northern and Southern Hemispheres. In fact, Earth's slightly varying orbital distance has virtually no effect on the weather. The real cause of the seasons is Earth's axis tilt, which causes the two hemispheres to take turns being tipped toward the Sun over the course of each year.

Why are Northern Hemisphere seasons are slightly more extreme than those of the Southern Hemisphere?

Most of Earth's land lies in the Northern Hemisphere, with far more ocean in the Southern Hemisphere. As you'll notice at any beach, lake, or pool, water takes longer to heat or cool than soil or rock (largely because sunlight heats bodies of water to a depth of many meters while heating only the very top layer of land). The Northern Hemisphere, with more land and less ocean, heats up and cools down more easily, which is why it has the more extreme seasons.

Does precession change the amount of axis tilt?

No. Note that precession does not change the amount of the axis tilt (which stays close to) and therefore does not affect the pattern of the seasons.

What does step 1 illustrate?

Step 1 illustrates the tilt of Earth's axis, which remains pointed in the same direction in space (toward Polaris) throughout the year. As a result, the orientation of the axis relative to the Sun changes over the course of each orbit: The Northern Hemisphere is tipped toward the Sun in June and away from the Sun in December, while the reverse is true for the Southern Hemisphere. That is why the two hemispheres experience opposite seasons. The rest of the figure shows how the changing angle of sunlight on the two hemispheres leads directly to seasons.

What does step 2 show?

Step 2 shows Earth in June, when the axis tilt causes sunlight to strike the Northern Hemisphere at a steeper angle and the Southern Hemisphere at a shallower angle. The steeper sunlight angle makes it summer in the Northern Hemisphere for two reasons. First, as shown in the zoom-out, the steeper angle means more concentrated sunlight, which tends to make it warmer. Second, if you visualize what happens as Earth rotates each day, you'll see that the steeper angle also means the Sun follows a longer and higher path through the sky, giving the Northern Hemisphere more hours of daylight during which it is warmed by the Sun. The opposite is true for the Southern Hemisphere at this time: The shallower sunlight angle makes it winter there because sunlight is less concentrated and the Sun follows a shorter, lower path through the sky.

How can we tell which day is going to be the summer solstice?

The June solstice occurs on the day on which the Sun follows its longest and highest path through the Northern Hemisphere sky (and its shortest and lowest path through the Southern Hemisphere sky). It is therefore the day on which the Sun rises and sets farthest to the north of due east and due west; it is also the day on which the Northern Hemisphere has its longest hours of daylight and the Sun rises highest in the midday sky.

Why do the seasons occur?

The answer is that the tilt of Earth's axis causes sunlight to fall differently on Earth at different times of year.

Where do the most extreme seasons occur?

The most extreme cases occur at the North and South Poles, where the Sun remains above the horizon for 6 months in summer and below the horizon for 6 months in winter.

What is true about zodiac sun signs?

The problem is that astrological Sun signs are based on the positions of the Sun among the stars as they were almost 2000 years ago. Because Earth's axis has moved about 1/13 of the way through its 26,000-year precession cycle since that time, astrological Sun signs are off by nearly a month from the actual positions of the Sun among the constellations today.

Our calendar keeps the solstices and equinoxes around the same dates each year, but the constellations associated with them change gradually over time. Why?

The reason is precession , a gradual wobble that alters the orientation of Earth's axis in space.

Why does the Earth precess?

The spinning (rotating) Earth precesses because of gravitational tugs from the Sun and Moon. Earth is not quite a perfect sphere, because it bulges at its equator. Because the equator is tilted 23.5 degrees to the ecliptic plane, the gravitational attractions of the Sun and Moon try to pull the equatorial bulge into the ecliptic plane, effectively trying to "straighten out" Earth's axis tilt. However, like the spinning top, Earth tends to keep rotating around the same axis. Gravity therefore does not succeed in straightening out Earth's axis tilt and instead only makes the axis precess. T

How is it summer in the Northern Hemisphere but winter in the Southern Hemisphere?

The sunlight angle gradually changes as Earth orbits the Sun. At the opposite side of Earth's orbit, Step 4 shows that it has become winter for the Northern Hemisphere and summer for the Southern Hemisphere. In between these two extremes, Step 3 shows that both hemispheres are illuminated equally in March and September. It is therefore spring for the hemisphere that is on the way from winter to summer, and fall for the hemisphere on the way from summer to winter.

True or false: Northern Hemisphere seasons are slightly more extreme than those of the Southern Hemisphere.

True. How do we know? Weather records.

How can we mark the dates of the equinoxes and solstices?

We can mark the dates of the equinoxes and solstices by observing changes in the Sun's path through our sky. The equinoxes occur on the only two days of the year on which the Sun rises precisely due east and sets precisely due west; these are also the two days when the Sun is above and below the horizon for equal times of 12 hours (equinox means "equal night").

Do seasons differ in equatorial regions?

Yes. Seasons also differ in equatorial regions, because the equator gets its most direct sunlight on the two equinoxes and its least direct sunlight on the solstices. As a result, instead of the four seasons experienced at higher latitudes, equatorial regions generally have rainy and dry seasons, with the rainy seasons coming when the Sun is higher in the sky.

December solstice (winter solstice in Northern Hemisphere)

occurs around December 21 and is the moment when the Northern Hemisphere receives the least direct sunlight.

June solstice (summer solstice in Northern Hemisphere)

occurs around June 21 and is the moment when the Northern Hemisphere is tipped most directly toward the Sun and receives the most direct sunlight

March equinox (spring or vernal equinox in Northern Hemisphere)

occurs around March 21 and is the moment when the Northern Hemisphere goes from being tipped slightly away from the Sun to being tipped slightly toward the Sun.

September equinox (fall or autumnal equinox in the Northern Hemisphere)

occurs around September 22 and is the moment when the Northern Hemisphere first starts to be tipped away from the Sun.