Perihelion - What's it All About?

At this time of year, Earth is rapidly approaching perihelion--its point of closest approach to the sun. This is a consequence of Earth's orbit being elliptical, not circular.

Take a look at this diagram:

In the above diagram, the Earth is a blue ball; the Sun is a yellow ball.  Earth is travelling in the counter-clockwise direction. As you can see, we're rapidly approaching the point of closest approach (A). The opposite point, aphelion, is at point (B). The Vernal Equinox, which marks the first day of spring, is at (C); and the autumnal equinox, marking the first day of fall, is at (D).

Now, interestingly enough, the speed at which our planet moves is inversely proportional to its distance from the sun (I know that's not quite true; but it will suffice for now). Right now, our planet is moving faster than usual. This has consequences.

The first consequence is that the planet spends less time between the autumnal and vernal equinoxes (the left-hand side of the image), and more time between the vernal and autumnal equinoxes. You can confirm this by counting the days between the two equinoxes. Now, Earth's orbital eccentricity is small, and so is the effect; but it's there.

This also has the effect of advancing the equation of time (basically put, the difference between the actual time, and the time a sundial would tell you). Right now, Earth is moving faster than usual; this has the effect of pushing both sunrise and sunset 'backward.' And so the equation of time is increasing in the negative direction (you'd have to subtract from the sundial time). Around the vernal equinox, this situation will reverse itself.

A related consequence is that, while the longest and shortest days of the year remain on the solstices, the dates of earliest and latest sunrise and sunset are offset. We've already passed our earliest sunset of the year, for example; but sunrise will continue to get later into late December/early January.

One other consequence to be noted, here: because perihelion falls shortly after the southern solstice (winter in the Northern Hemisphere), Southern-Hemispheric summers are slightly warmer, and the winters slightly colder.

That's enough for now.

-Bill