Explaining the science behind, "What time is it?"

Uploaded by PennState on 24.05.2012

What time is it? is such a simple question, but like a lot of simple
questions, when you dig down into the details you will be amazed at just how complicated
it can be. I'm sure that when I ask you, you might just
pull out your watch or your cellphone and you might tell me that
it’s 12:00 noon.
If we were in Philadelphia and I asked you, you would still say 12:00. Same
thing if we were in Pittsburgh. But is it really 12:00 in western, central,
and eastern PA, all at the same time? Not really, but to make our lives
simple, we have agreed that in big bands on the
Earth called "time zones," we would all set our clocks to the same time.
If you really want to know when it is noon, you want to know when the sun is
exactly due south from your position. If you want to use astronomer’s terminology
for this, we call this, "transiting the meridian," so the meridian
is an imaginary line that stretches from due south, goes directly
overhead and ends due north.
So true noon, which we call, "solar noon" happens each day when the Sun is on
your meridian. The thing is, if you measured that with a sundial like this
one, you would find that solar noon according to the sundial might happen
at 12:13, 11:58, or even 1:07 on your watch! So what’s going on?
Well, after you account for the hour of daylight savings time there would still
be a difference of 13 minutes between our clocks and the sundial.
The second correction is the time zone
correction. We here in State College are not in
the middle of our time zone. But we set our clocks to the same time they do.
So when the Sun is on their meridian, it is still about 11 and a half minutes
until it will be on our meridian. So this is the "longitude correction," and
we will always be off from solar noon with our sundial by this same 11 and a
half minutes each day. Still, though, we have one last
problem to sort out -- there is still about a minute and a half
difference. Well, this one may be the biggest surprise of them all.
If you take "one day" as being the time the Earth rotates around one time, you
would expect that the time from noon on one day to noon on the next day is
always one day, or 24 hours long. Well, it isn’t! Sometimes it takes a little
longer than 24 hours for the Sun to be on the meridian, and sometimes a
little less than 24 hours. So over the course of the year, sometimes a sundial
will run slow and sometimes run fast compared to a clock.
You can draw a shape called an "analemma," that shows where the Sun
will be on the sky exactly 24 hours later each day from one day to the
next. This effect can be up to roughly 15 minutes of time different from
"noon" each day. During spring, it’s only a minute and
a half, but on Feb 6, solar noon by the sundial occurred at 1:25 pm on our
clocks! Real simple,... though, we’ve got to
figure out something to do with those leap years...
For Penn State I’m Astronomer Chris Palma.