The Seasons

Key idea: Heating of a planet's surface and atmosphere varies over the course of a year because of the tilt of the rotation axis of the planet and the orbit of the planet around the Sun. Seasonal effects result from the periodic variation of surface and atmospheric heating.

Seasonal Changes on North Pole of Mars Astronomical tidbit: This series of three images of the North Pole of Mars was taken with the Wide Field and Planetary Camera 2 aboard the Hubble Space Telescope between October, 1996 and March, 1997. The Hubble images have been projected to appear as if they were taken from directly above the Pole when in fact they were taken from orbit around Earth. The first image was taken in early Spring in the northern hemisphere of Mars, and shows the northern ice cap extending from the Pole down to approximately 60 degrees north latitude. The second photograph was taken in the middle of the northern Spring. Due to increased heating in the northern hemisphere, carbon dioxide in the cap has begun to sublimate and cycle back into the atmosphere. The cap has receded to approximately 70 degrees north latitude. Atmospheric pressure on Mars increases at this time due to the increased mass of carbon dioxide gas in the atmosphere. The final photo was taken near the Martian summer solstice in the northern hemisphere. The northern polar cap has further sublimated at this time leaving a small core of water ice.

All planets and satellites rotate about an axis which goes through their North and South Poles. The energy which produces the rotation remains from the time that the Solar System was formed. On Earth, it takes one day for our planet to complete one rotation with respect to the Sun. Some planets rotate faster than Earth and some rotate slower. The day is longer on the planets which rotate more slowly and the day is shorter on the planets which rotate more quickly than Earth.

The moon is gravitationally locked to the Earth so that the same side always faces the Earth. Nevertheless, the Moon completes one rotation on its axis each month as it completes one cycle through all of its phases.

All of the planets orbit around the Sun. On Earth, it takes one year to complete an orbit around the Sun. Planets which are closer to the Sun than Earth orbit more quickly than we do. The length of the year is shorter on those planets. Planets which are further from the Sun than the Earth take longer to complete an orbit than we do. On these planets, the year is longer than it is on Earth. All of the planets orbit the Sun in roughly the same plane. This is called the plane of the ecliptic. If you look at the sky after dark or before dawn, you will notice that you can almost draw a straight line across the sky through the planets which are visible. This line is in the plane of the ecliptic.

Activity: For mathematically advanced students, use Kepler's Law to determine the length of the year given mean distance from the Sun for planets in the Solar System.

The rotation axes of the planets are not generally perpendicular to the plane of the ecliptic. This means that the planets are not oriented at right angles to the plane in which they orbit the Sun. You will soon find out that if the planets were oriented at right angles to the plane of the ecliptic then there would not be any seasons!!! The rotation axis of the Earth is tilted by 23.5 degrees with respect to a right angle to the plane of the ecliptic. That is why most globes of the Earth are tilted so that the North and South Poles are not located exactly at the top and bottom. There is a large variation in the amount that each of the planets are tilted, but rest assured that they are all in fact tilted by some amount.

The tilt of the poles of any planet does not change much over the short term (hundreds of years anyway). For example, the Earth's axis points toward Polaris (the so-called "North Star") and will do so for a few more centuries. As planets orbit around the Sun, however, the direction of the rotation axis does change with respect to the Sun. This causes the seasons!!!

Demonstration: Carry a globe (with tilted rotation axis!) around a central point and notice how the orientation of the rotation axis changes with respect to the central point. Better yet, hold a flashlight at the central point and illuminate the globe as it completes its orbit. Try to determine the seasons on the globe as it makes its orbit, and notice that any spinning of the globe does not change the direction of its rotation axis.

In the northern Summer (southern Winter), the North Pole is tilted toward the Sun as the Earth makes its orbit. At this time of year, the Sun shines more directly on the northern hemisphere than the southern hemisphere and the days are also longer in the north than they are in the South. You have certainly noticed that the days are longer in Summer, but you have also probably noticed that the Sun also appears higher in the sky at that time of year. The moment when the North Pole is tilted most directly at the Sun (remember that it is never tilted exactly at the Sun) is called the summer solstice.

In the northern Winter (southern Summer), the North Pole is tilted away from the Sun. The moment when the North Pole is tilted furthest away from the Sun is called the winter solstice. At this time, the northern hemisphere receives the least energy from Sun and the days are shortest. In the northern Spring and Fall, the Poles are tilted in a direction perpendicular to the Sun. The exact moments that the rotation axis is perpendicular to the Sun are called the vernal and autumnal equinoxes. On the dates of the equinoxes, there is 12 hours of daylight everywhere on Earth.

The orbits of the planets around the Sun are not necessarily circular but are elliptical. As a result, the distance between the planet and the Sun changes over the course of a year. The closest point in a planet's orbit around the Sun is called perihelion and the furthest point from the Sun is called aphelion. In addition to the seasonal effects due to the pointing of the Poles, there can also be signficant effects due to changing distance from the Sun. As you have learned in a previous lesson, this is because the Solar flux which is incident on a planet decreases as the distance to the planet increases. At Mars, the temperature differences from perihelion to aphelion can be as much as 55 degrees Fahrenheit.

You have learned that the tilt of a planet's rotation axis does not change much over relatively short periods of time. The actual tilt can change over long time periods, however, just as the direction of its rotation axis can change while a gyroscope is spinning. The change in the direction of Earth's rotation axis is called the precession of the equinoxes. The Earth precesses with a period of about 26,000 years. In that amount of time our rotation axis will point pretty much where it does today. In half that time, however, the summer solstice will be in December and the winter solstice will be in June!!! The tilt angle itself can also change over long periods. This is the case on Mars which does not have a large satellite like the Moon to stabilize the angle which its rotation axis is tilted. The tilt angle of the rotation axis on Mars varies between 15 and 35 degrees with a period of approximately 125,000 years. When the angle is large, the poles receive more solar energy than they do now (when the tilt is about 25 degrees) and when the angle is smaller the poles receive less solar energy. Why do you think that is the case???

Last updated: November 17, 1999
Joe Twicken /
Rob Wigand