Tonight, or before dawn tomorrow, our planet Earth will reach perihelion – its closest point to the sun for the year. This annual event will take place on January 4, 2015 at 6:37 UTC (01:37 EST). The word perihelion is from Greek roots peri meaning near, and helios meaning sun.
Earth is closest to the sun every year in early January, when it’s winter for the Northern Hemisphere. We’re farthest away from the sun in early July, during our Northern Hemisphere summer.
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’s not a huge change in distance. It’s not enough of a change to cause the seasons on Earth.
Despite what many think (and there’s every reason to suppose this), Earth’s distance from the sun isn’t what causes the seasons. On Earth, because our orbit is so close to being circular, it’s 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. The day of maximum tilt toward or away from the sun is the December or June solstice.
Though not responsible for the seasons, Earth’s closest and farthest points to the sun do affect seasonal lengths. When the Earth comes closest to the sun for the year, as around now, our world is moving fastest in orbit around the sun. Earth is rushing along now at 30.3 kilometers per second (almost 19 miles per second) – moving about a kilometer per second faster than when Earth is farthest from the sun in early July. Thus the Northern Hemisphere winter (Southern Hemisphere summer) is the shortest season as Earth rushes from the solstice in December to the equinox in March.
In the Northern Hemisphere, the summer season (June solstice to September equinox) lasts nearly 5 days longer than our winter season. And, of course, the corresponding seasons in the Southern Hemisphere are opposite. Southern Hemisphere winter is nearly 5 days longer than Southern Hemisphere summer.
It’s 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.
Bottom line: In 2015, Earth’s closest point to the sun – called its perihelion – comes on January 4 at 6:37 UTC (01:37 EST).