Alan Stahler: Days grow longer, but sunrise still comes later |

Alan Stahler: Days grow longer, but sunrise still comes later

Alan Stahler
The moon's orbit around Earth is an ellipse; so is Earth's orbit around the sun.
Courtesy of NASA |

The December solstice — shortest day of the year — came and went two weeks ago. As the days grew shorter, leading up to the solstice, those of us getting up before the sun had to wait longer and longer — later and later — for the sun to rise.

Now we’ve passed the solstice, and the days are growing longer … but the sun is still rising later, day after day.

Last Monday’s supermoon was a treat — definitely larger than usual. If you missed it, there will be another supermoon on the last day of the month, during which the moon will suffer total eclipse. Try to not miss that one.

The moon circles the earth in an elliptical orbit (Don’t confuse an ellipse with an oval — an oval is egg-shaped — wider at one end than the other).

A circle is a special kind of ellipse. Most ellipses look like circles that have been stepped on — squashed circles.

The Earth is not at the “center” of the moon’s elliptical orbit — we’re more toward one end. This means that the moon comes closer to us in some parts of its orbit than in others.

When the moon is full at the same time that it is closest, we enjoy a supermoon.

Earth, too, orbits the sun in an elliptical orbit, bringing us sometimes closer to the sun, sometimes farther away.

An object in orbit is always falling — the sun is constantly trying to pull us into itself. Fortunately, we’ve got some sideways motion, and the result is a compromise: Even as the sun tries to pull us down, inertia moves us sideways, and we wind up falling, not into the sun, but around it.

One of the rules of falling is that, the closer you are to the thing pulling you down, the faster you go.

Testing orbits

Fling a ball upward. It leaves your hand fast, but slows as it rises. When its upward velocity drops to zero, it begins to fall, faster and faster, until you catch it.

An object in orbit is falling, so the closer we are to the sun, the faster we go. When Earth is closest to the sun, in early December, we’re moving at our fastest clip.

The band is playing, the lights are flashing, you and your partner are dancing your brains out. You spin around, stop, and there she is in front of you. Spin again, stop, and there she is in front of you again.

Now she starts dancing around you. You spin like before, stop … but she’s not there. You’ll have to spin just a bit more, to face her again.

The faster your partner dances around you, the farther you’ve got to spin to catch up with her.

It’s pretty well proven that Earth circles the sun — not vice-versa — but, sometimes, it’s easier to pretend that everything goes around us.

Day after day, the sun dances around us, so from one day to the next, we’ve got to turn just a little bit more, to see the sun rise over the horizon.

When we’re closest to the sun — as we were two weeks ago — the sun dances around us at her fastest, so we’ve got to turn more than usual … so the sun rises later than the day before — even after the solstice, the shortest day of the year.

Now the days are, indeed, growing longer to make up for the later sunrise, the sun is setting later, too. Mid-January, the sun will again start rising earlier and earlier.

There’s an equation that takes all the Earth’s motions — its speed-ups and slow-downs, and its wobbles — and computes where the sun will be — and when it will rise or set — at any time throughout the year. It’s the equation of time.

Alan Stahler’s science programs can be heard on KVMR (89.5 FM). He brings an enjoyment of science and nature to students of all ages, and may be reached at

Start a dialogue, stay on topic and be civil.
If you don't follow the rules, your comment may be deleted.