Happy Aphelion Day!

So did you notice that it was a little cool yesterday, perhaps compared to last January? Well, of course, those of us in the northern hemisphere would of course say no, it was colder last Winter. But yesterday the Earth passed aphelion - the point in its orbit farthest from the sun. I noted last January that we were passing the near point, perihelion at that time. The elliptical nature of the Earth's orbit is a little over 3%, but is not responsible for the seasons. It happens to be warmer near aphelion for us because in the northern hemisphere, the sun is shining more directly down on us and for longer days. It happens to be cooler when we're closer to the sun in Winter because of the shallower angle of sunlight and shorter days - again, in the northern hemisphere. The image above was taken of the sun this afternoon, and shows part of the disk at full camera resolution through the TEC 140 telescope.  North is approximately up, and shows active region (AR) 2381 that is transiting the Sun's disk now.

Since I used the same camera and telescope combination last January, we can compare the apparent change of the Sun's diameter. I've never seen this demonstrated, though I did it for the Moon last year. In that case, the difference in the moon's size between apogee and perigee is 14%, so is much more dramatic. In the case of the Earth's orbit around the Sun, 3.4% is much less dramatic, but easily visible here at left. By chance there happened to be active regions at the same solar longitude when both images were taken - AR 2381 in the top image taken today, and AR 2253 on the bottom image from 3 January. An alternative display is shown at right, where both were aligned in Photoshop with an opacity of 50% so both images can be compared, around their edges anyway...

I was actually a day late in my aphelion imaging session, though yesterday was a much cloudier day without much chance to image and the difference of a day doesn't amount to much, distance wise... Rounded off to the nearest 1000 miles in the left image, they reflect both the aphelion/perihelion distances and the distance when the images were taken...

Even though the change in apparent solar diameter is small, it was neat to see the difference. While the Moon's change in apparent size might be detected by eye over the course of a couple weeks, you have to go search for the Sun's change - not a readily observed difference!