Courtesy of EarthSky
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When 2011 began on January 1, our planet Earth was very close to its perihelion – its closest point to the sun for the year. That closest point will come tomorrow. In 2011, Earth will be closest to the sun on Monday, January 3 at 19 hours Universal Time (12 p.m. Mountain Time).
Earth is closest to the sun every year in early January, when it is winter for the northern hemisphere. We are farthest away from the sun in early July, during our northern hemisphere summer. You can see that Earth’s distance from the sun isn’t what causes the seasons.
Earth is about 5 million kilometers – or 3 million miles – closer to the sun in early January than it will be in early July. That is not a huge change in distance. On Earth, it is mostly the tilt of our world’s axis that creates winter and summer. In winter, your part of Earth is tilted away from the sun. In summer, your part of Earth is tilted toward the sun.
Though not responsible for the seasons, Earth’s closest and farthest points to the sun do affect the lengths of the seasons. When the Earth comes closest to the sun for the year, as now, our world is also moving fastest in orbit around the sun. Earth is rushing along now – moving about a mile per second faster than our average speed in orbit of 18 miles per second. Thus, the northern hemisphere winter – and southern hemisphere summer – is the shortest season as Earth rushes from the winter solstice in December to the March equinox.
In the northern hemisphere, the summer season (June solstice to September equinox) lasts nearly 5 days longer than our winter season. Of course the corresponding seasons in the southern hemisphere are opposite. Southern hemisphere winter is nearly 5 days longer than southern hemisphere summer.
It is all due to the shape of Earth’s orbit. The shape is an ellipse, like a circle someone sat down on and squashed. The elliptical shape of Earth’s orbit causes the variation in the length of the seasons – and brings our closest point to the sun, this year on January 3.
Written by Bruce McClure