Aphelion and perihelion

¡SkyCaramba! Weekly astronomy blog for the week ending January 12, 2013

Three planets are at noteworthy points in their orbits around the sun this month. Earth and Mars are at perihelion. Mercury is at aphelion. Nothing particularly special happens to them as they move through space during these times. These orbital points are noteworthy because we can use them to explain planetary orbits.

Perihelion is the point in orbit closest to the sun. Aphelion is the point farthest away. If the planets’ orbits were perfectly circular, there would be no perihelion and aphelion. Each one would always be the same distance from the sun. Astronomers have known for several centuries, planets orbit in ellipses.

How much of an ellipse a planet orbits in is called its eccentricity. An eccentricity value of 0 would describe a perfect circle. In other words, there would be zero deviation from being a circle. If you squash the circle, eccentricity rises. It becomes more obviously an ellipse. When eccentricity reaches 1, it’s actually not an ellipse anymore either. Rather, it’s a straight line.

Earth’s eccentricity is 0.0167. That’s slightly elliptical. Earth is about 147 million km (91.4 million miles) from the sun at perihelion. At aphelion, it’s 152 million km (94.5 million miles) away. Each number differs by 1.67% from the average 149.5 million km (93 million miles). Earth reached perihelion this year on January 2.

Mars has an orbit that takes about two years, so it doesn’t go through perihelion every year. It just so happens the red planet will be at perihelion on the 24th. Its eccentricity is 0.0934. So we should expect its closest and farthest orbital points to vary 9.34% from average. Calculate it for yourself. Mars will be 206.6 million km (128.4 million mi) from the sun on the 24th. Next Jan. 3rd, it will be 249.2 million km (154.9 million mi) away.

Mercury is the most eccentric planet orbiting the sun, if you exclude Pluto from your list of planets as the International Astronomical Union has done. Mercury’s eccentricity is 0.2056. It will be 69.9 million km (43.4 million mi) from the sun on the 4th when it’s at aphelion. It will zoom in to a distance of 45.9 million km (28.5 million mi) at perihelion on February 17th.

The planets don’t have perfectly circular orbits because numerous forces have acted on them at various times for as long as they’ve been in existence. Think about how the planets formed. If some material is hurling in the general direction of the sun but not directly toward it, it will end up in an elliptical orbit if it ends up in an orbit at all. When lots of pieces of space rock going in all sorts of directions finally come together to form planets, their combined direction and momentum are not likely to leave them in perfectly circular orbits.

There’s your astronomy lesson for this week. ¡SkyCaramba!