The moon goes to extremes

SkyCaramba weekly astronomy blog for the week ending April 28, 2012

During the next few weeks, the moon will go from being its farthest from us all year, to its closest, to its farthest again. 

The moon’s orbit isn’t a perfect circle. A few artificial satellites have perfectly circular orbits, but none of the natural ones discovered so far do. During their courses around their parent bodies, natural satellites get closer and then farther away. 

The farthest point in the moon’s orbit around the earth is called apogee. The closest point is called perigee. The gee part comes from geo, a root word for Earth. There’s an apogee and a perigee every orbit. The distance from the center of the earth to the center of the moon changes by around 50,000 km from apogee to perigee or vice-versa. 

The earth is also in a slightly non-circular orbit around the sun. The root word for sun being helios in the astronomer’s word system, the earth is farthest from the sun at aphelion and closest to it at perihelion. The fact that the sun, moon, and earth all exert gravitational forces on each other means the earth and moon aren’t orbiting in perfect ellipses either. 

Earth has more mass than the moon. If you could weigh Earth and the moon in the same gravitational field, Earth would weigh more. And something with more mass exerts more gravitational attraction. So while the moon’s gravity tugs on the earth, Earth’s gravity tugs more on the moon. Because of the moon’s pull on the earth, Earth’s path around the sun is actually a wiggly ellipse. 

And while Earth makes its wiggly elliptical trek around the sun, it gets tugged a little more here or a little less there. Earth’s speed changes slightly throughout this do-se-do. So as the moon goes from apogee to apogee, it’s a little closer to Earth at some apogees than others. This month’s and next month’s apogees are the farthest of the year. And the perigee of May is the closest. 

The moon at perigee and apogee and at the same phaseOn April 22, 2012 at 14:03 UT, the moon will be 406,500 km from Earth. It’ll be the same distance again on May 19 at 16:28. All other apogees this year are a little closer than that. The moon’s closest perigee is on May 6 at 03:40 when it will be 356,900 km away. 

So how much bigger or smaller will the moon look at these extremes? It’s hard for us to notice the difference when the moon’s phase is different and it’s been two weeks. But photography enables us to make side by side comparisons. When pictures are taken with the same equipment at the same settings and with the moon at the same phase, we can see how the moon does look slightly smaller at apogee. 

For the April 22 apogee, the moon’s disk would take up a little less than 1,800 seconds of arc in our sky. It’ll be a very thin crescent just after new moon, so you won’t have much opportunity to see it. For the May 6 perigee, which happens to be a full moon, it takes up a little more than 2,000 second of arc. The moon’s disk will look 11% bigger. 


Here are a few web sites that explain the above topic more.