2 (f-a) December Solstice
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If Earth’s axis were straight up and down relative to its orbit, there would be no seasons. Earth has seasons because its axis is tilted as it revolves around the sun.
Earth’s axis is always tilted at an angle of 23.5° from the vertical. As Earth revolves around the sun, the north end of its axis is tilted away from the sun for part of the year and toward the sun for part of the year.
Summer and winter are caused by Earth’s tilt as it revolves around the sun. The change in seasons is not caused by changes in Earth’s distance from the sun. In fact, Earth is farthest from the sun when it is summer in the Northern Hemisphere.
In June, the north end of Earth’s axis is tilted toward the sun. In the Northern Hemisphere, the noon sun is high in the sky and there are more hours of daylight than darkness. The combination of direct rays and more hours of sunlight heats the surface more in June than at any other time of the year. It is summer in the Northern Hemisphere.
At the same time south of the equator, the sun’s energy is spread over a larger area. The sun is low in the sky and days are shorter than nights. The combination of less direct rays and fewer hours of sunlight heats Earth’s surface less than at any other time of the year. It is winter in the Southern Hemisphere.
In December, people in the Southern Hemisphere receive the most direct sunlight, so it is summer there. At the same time, the sun’s rays in the Northern Hemisphere are more slanted and there are fewer hours of daylight. So it is winter in the Northern Hemisphere.
The sun reaches its greatest distance north or south of the equator twice each year. Each of these days, when the sun is farthest north or south of the equator, is known as a solstice (sohl stis). The day when the sun is farthest north of the equator is the summer solstice in the Northern Hemisphere. It is also the winter solstice in the Southern Hemisphere. This solstice occurs around June 21 each year. It is the longest day of the year in the Northern Hemisphere and the shortest day of the year in the Southern Hemisphere.
Similarly, around December 21, the sun is farthest south of the equator. This is the winter solstice in the Northern Hemisphere and the summer solstice in the Southern Hemisphere.
Equinoxes
Halfway between the solstices, neither hemisphere is tilted toward or away from the sun. This occurs twice a year, when the noon sun is directly overhead at the equator. Each of these days is known as an equinox, which means “equal night.” During an equinox, day and night are each about 12 hours long everywhere on Earth. The spring equinox occurs around March 21 and marks the beginning of spring in the Northern Hemisphere. The fall equinox occurs around September 22. It marks the beginning of fall in the Northern Hemisphere.
Earth’s axis is always tilted at an angle of 23.5° from the vertical. As Earth revolves around the sun, the north end of its axis is tilted away from the sun for part of the year and toward the sun for part of the year.
Summer and winter are caused by Earth’s tilt as it revolves around the sun. The change in seasons is not caused by changes in Earth’s distance from the sun. In fact, Earth is farthest from the sun when it is summer in the Northern Hemisphere.
In June, the north end of Earth’s axis is tilted toward the sun. In the Northern Hemisphere, the noon sun is high in the sky and there are more hours of daylight than darkness. The combination of direct rays and more hours of sunlight heats the surface more in June than at any other time of the year. It is summer in the Northern Hemisphere.
At the same time south of the equator, the sun’s energy is spread over a larger area. The sun is low in the sky and days are shorter than nights. The combination of less direct rays and fewer hours of sunlight heats Earth’s surface less than at any other time of the year. It is winter in the Southern Hemisphere.
In December, people in the Southern Hemisphere receive the most direct sunlight, so it is summer there. At the same time, the sun’s rays in the Northern Hemisphere are more slanted and there are fewer hours of daylight. So it is winter in the Northern Hemisphere.
The sun reaches its greatest distance north or south of the equator twice each year. Each of these days, when the sun is farthest north or south of the equator, is known as a solstice (sohl stis). The day when the sun is farthest north of the equator is the summer solstice in the Northern Hemisphere. It is also the winter solstice in the Southern Hemisphere. This solstice occurs around June 21 each year. It is the longest day of the year in the Northern Hemisphere and the shortest day of the year in the Southern Hemisphere.
Similarly, around December 21, the sun is farthest south of the equator. This is the winter solstice in the Northern Hemisphere and the summer solstice in the Southern Hemisphere.
Equinoxes
Halfway between the solstices, neither hemisphere is tilted toward or away from the sun. This occurs twice a year, when the noon sun is directly overhead at the equator. Each of these days is known as an equinox, which means “equal night.” During an equinox, day and night are each about 12 hours long everywhere on Earth. The spring equinox occurs around March 21 and marks the beginning of spring in the Northern Hemisphere. The fall equinox occurs around September 22. It marks the beginning of fall in the Northern Hemisphere.