Earth travels once around the Sun in one year. This is called revolution, and the tilt of Earth's axis produces the four seasons.
Basic Knowledge
Earth's orbital period is about 365.25 days, and its orbit is slightly elliptical (eccentricity 0.017). The direction of revolution is counterclockwise when viewed from the north, the same as rotation.
Earth's axis is tilted about 23.4 degrees relative to the orbital plane, and this is the main cause of the seasons. In summer, the Northern Hemisphere is tilted toward the Sun, so sunlight strikes it more directly and for a longer time, making it warmer. In winter, the opposite occurs: sunlight strikes at a slant and for a shorter time, making it colder.
Summer solstice (around June 21): the Sun's altitude is greatest and daytime is longest in the Northern Hemisphere. Winter solstice (around December 22): the Sun's altitude is smallest and daytime is shortest. Spring and autumn equinoxes: the Sun rises due east and sets due west, and day and night are nearly equal in length.
Revolution (the movement of one celestial body around another)
Earth's axis (Earth's rotational axis; tilted 23.4 degrees to the orbital plane)
Summer solstice (greatest solar altitude and longest daytime in the Northern Hemisphere; around June 21)
Winter solstice (smallest solar altitude and shortest daytime in the Northern Hemisphere; around December 22)
Spring and autumn equinoxes (day and night are nearly equal; the Sun rises due east and sets due west)
Solar-noon altitude (the angle above the horizon when the Sun is due south)
Further Study (Background and Significance)
At a location of north latitude φ, the solar-noon altitude at the spring and autumn equinoxes is 90-φ. At the summer solstice it is 90-φ+23.4, and at the winter solstice it is 90-φ-23.4. For example, in Tokyo (about 36 degrees north latitude), the values are 54 degrees at the spring and autumn equinoxes, 77.4 degrees at the summer solstice, and 30.6 degrees at the winter solstice.
If Earth's axis were not tilted, Earth would have no seasons and the same climate all year. Mars (about 25 degrees axial tilt) and Saturn (about 27 degrees) also have seasons, and the seasons on Mars last about twice as long as those on Earth because Mars has a longer orbital period.
Earth's axis also undergoes slight precession (a wobbling motion like a spinning top), causing the north celestial pole to change over a period of about 26000 years. Polaris is the current guide star, but in about 12000 years, Vega in Lyra will be the northern guide star.
- Orbital period = about 365.25 days
- Direction of revolution = counterclockwise when viewed from the north
- Axial tilt = 23.4 degrees
- Summer solstice = 6/21; winter solstice = 12/22; spring and autumn equinoxes = around 3/21 and 9/23
- Solar-noon altitude at the equinoxes = 90-latitude
- Summer solstice = 90-latitude+23.4
- Winter solstice = 90-latitude-23.4
Points of Caution (Easily Confused)
① The main cause of the seasons is Earth's axial tilt, not the distance between Earth and the Sun (Earth is closer at perihelion in January, during Northern Hemisphere winter). ② The summer solstice has the longest daytime, and the winter solstice has the shortest. “Summer is hot” occurs 1–2 months later because of the heat capacity of the ground. ③ The seasons are reversed in the Southern Hemisphere. ④ Remember the solar-noon altitude formulas (latitude φ; summer solstice = 90-φ+23.4).
Practice
- By how many degrees is Earth's axis tilted relative to the orbital plane?
- Around when is the summer solstice in the Northern Hemisphere?
- Find the solar-noon altitude on the spring equinox at a location at 36 degrees north latitude.