Friday, January 02, 2009

A great question!

Hello Scott- Love your blog and your book. Question: According to US Naval Observatory charts, we're getting more daylight after winter solstice since sunset is later every day. But sunrise is not earlier until January 6, at least here in Flint Michigan. Why is that?

This question appeared in the comments on a former post. It's such a good question I thought I'd answer it here. Thanks to dfrank in Flint MI for the question. Now here goes!

This is a tough one! There are two effects going on. First there is the equation of time. Due to our varying speed in our orbit (mainly) noon on a clock will differ from noon as measured by a sun dial. Sometimes the sun is ahead and sometimes behind depending on the time of year. In late December into January the dial shows noon before the clock does. So the sun is 'slow' and sunrise would occur later and sunset later.

When the graph is above the line the sundial is ahead of the sun and when below it is behind.

The other effect is that sunrise should be ocuring earlier and sunset later due to the changing of the seasons. The sun is getting higher in the sky. So there is a bit of time when one effect is trying to make sunrise later and the other effect is trying to make sun rise earlier. These can then cancel. For the sunset though, both effects are making the sun set later. So that's why until about Jan 6, sunrise in Flint Michigan stays about the same while sun set happens later and later.

Here's a pretty good link to this effect

Here's one about the equation of time


Anonymous said...

The guy who asked that question is my brother, from Flint. And I can't begin to understand your answer. But I'm sure he does!

Rich P

Anonymous said...

Thanks, Scott. I was aware that solar noon varied throughout the year but had not considered how it would affect the timing of sunrise and sunset.

Dan in Flint