PS 163Q Lied To Me

Somewhere in grade school one my teachers lied to me!

Yours did too. They taught us things about the first day of spring and fall that weren’t true.

Your calendar might indicate fall begins Tuesday. It actually happens Monday here in the US. Tuesday’s correct for everyone outside the Americas.

Every first day of fall and spring I think of a little esoteric tidbit all meteorologists understand.

Somewhere in grade school one my teachers lied to me! Yours did too. They taught us things about the first day of spring and fall that weren’t true.

Your teacher explained how at the equinox day and night are equal. And it makes total sense.

It’s just not so.

SUN
Begin civil twilight 6:14 a.m.
Sunrise 6:39 a.m.
Sun transit 12:44 p.m.
Sunset 6:48 p.m.
End civil twilight 7:12 p.m.

Those numbers are for Irvine. About nine minutes more day than night. That’s not a little, teach.

Here’s New Haven,

SUN
Begin civil twilight 6:12 a.m.
Sunrise 6:39 a.m.
Sun transit 12:44 p.m.
Sunset 6:49 p.m.
End civil twilight 7:16 p.m.

Ten minute difference there!

So, what’s going on? Where did the staff of PS 163Q go wrong?

Sunrise to sunset is the time from the first light of day to the last tiny solar dot.

The next two sentences are complex, so take your time and re-read if you need to. Be brave. I promise you’ll get it.

Day doesn’t end when the portion of the Sun that rose first, sets. You have to add the time it takes for the diameter of the Sun to go by.

Think of it as the front end rising and back end setting.

If we had no atmosphere that would be it. At the equinox, day would be a few minutes longer than night.

We do have an atmosphere!

Near the horizon sunlight makes its longest possible trip through the atmosphere. The Sun’s light is refracted, meaning it gets bent a little.

At sunset the Sun is actually a few degrees below the horizon. Refraction is bending the light up. Seeing it on the horizon is really an optical illusion!

Add the extra time for the diameter of the Sun plus refraction and there’s more daylight every day! It’s just easier today when things ‘should’ be equal.

And whatever you do, don’t get me started on balancing eggs. Ugh.

3 thoughts on “PS 163Q Lied To Me”

1. Melanie says:

When I saw the headline, I was sure it was about the egg-balancing myth -shocking to learn something else is also untrue about the equinox(es)! Or is it equinoxi?

2. dave says:

Geoff

Most interesting, science lesson.

-dave

PS I sure miss your prior, on air science segments

3. Mike Robinson says:

You are like me, a northern hemisphere boy. In the southern hemisphere, the March equinox is autumn.