Photo credit: Solar Dynamics Observatory (SDO): CLICK photo for larger size and look below for information and links.

Happy perihelion everyone; today (January 4, 2014) you will be as close to the Sun as you will be all year long at a distance of 0.98 AU (91,096,691 mi./146,605,913 km.).
Today you are a whopping 3 million miles (5 million kilometers) closer to the Sun than you will be on July 3rd when the Earth reaches aphelion (Furthest point from the Sun in its orbit).
To be fair, with an average distance of 92,955,807 mi. (149,597,870 km.) that’s not much of a change at all. Thanks to Kepler’s 1st Law; the planets orbit in an elliptical orbit but the Earth’s orbit is pretty darn close to a prefect circle. From Earth’s furthest point in July to its closest today there’s only about a 3% change in total distance from the Sun.

The term perihelion is Greek and it breaks down like this: “Helios” = Sun and “peri” = near. In July “apo” for aphelion = away from.

A pretty obscure fact is that due to precession, roughly every 58 years perihelion gets one day later (Though there are year to year fluctuations) so if we’re patient and wait about 10,000 years perihelion will occur during summer in the northern hemisphere whereas it falls in the southern hemisphere summer today…..not that it changes anything.

If you live in the southern hemisphere it’s possible that you may be thinking that’s the reason why it’s summer. Conversely; if you’re in the northern hemisphere you may be questioning why it’s so cold if were at our closest point to the Sun. The facts are that perihelion and aphelion have almost nothing to do with the seasons……let’s take a quick look at this.

Seasons change because of what’s called axial tilt which for Earth is 23.45 degrees. This tilt, combined with Earth’s orbit is what actually creates the seasons. In the winter, the northern hemisphere is tilted away from the Sun which means the southern hemisphere is tilted toward the Sun placing them in summer. Six months later this changes completely and the roles are reversed. It’s also important to realize that the Earth doesn’t actually tilt back and forth every season, it stays relatively the same. What happens is, as the Earth orbits the Sun; that 23.45 degree tilt places the Earth in a position that changes which hemisphere gets more sun light.
Just to illustrate; even though the earth has moved 186 million miles to the opposite side of the Sun in 6 months time, Polaris is still the North Star so you know the tilt hasn’t moved, just the Earth has.
Try this the next time you have a school room globe and a light handy to simulate the Sun. Start by placing the Globe with the North Pole facing away from the Sun to simulate northern hemisphere winter. Now, while keeping the North Pole pointed in the same direction place the Earth on the exact opposite side of the “Sun”. Now you can see that the Northern Hemisphere gets the majority of the Sun’s light to simulate summer and the poles are still pointing in the same directions. This illustrates that it’s really not the Earth that tilts but in fact the earth IS TILTED; thus, the change from one side of the Sun to the other places the different hemispheres in a position to receive more radiation from the Sun.

If you’re even more astute you may say to yourself, “Well in the northern hemisphere the winter solstice in on December 21st but the coldest months of the year are in January and February. And summer solstice falls on June 21st while the hottest months of the year fall on July and August…..what gives?!!”
Great observation, and there’s an answer for that as well. It’s called the Lag of the Seasons and it’s basically because the earth is a gigantic heat sink. Think of it as your diet, if you change your diet, things don’t change immediately but instead slowly, over months. Or an oil tanker, to change direction or to speed up or slow down you have to make your corrections before you actually need the result because there’s a large lag time between the manual action and that desired result. The same applies to the Earth, the weeks of long days during late May and June finally heat the Earth’s atmosphere and oceans and the result is felt in hot July and August months. In the winter the darkest nights of the year in late November, December finally chill the atmosphere and oceans enough create the coldest of cold months in January and February.
Try to pay attention to it this year, the coldest months lag behind the darkest days and the hottest months lag behind the longest days.

NASA SDO page for this photo: http://sdo.gsfc.nasa.gov/gallery/main/item/157

In–The–Sky page on this event: http://in-the-sky.org/news.php?id=20140104_09_100

Earth Sky article on this event: http://earthsky.org/tonight/earth-comes-closest-to-sun-every-year-in-early-january

Image | This entry was posted in Astronomical Events, Astronomy (Learning), Images, News, Solar System, Stars (Non-Sun Related) and tagged , , , , , , , , , , . Bookmark the permalink.

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