(Submitted July 14, 1997)
When is the Sun at its highest point and how did you determine the answer?
Thank you very much for your question about the Sun, it was a pleasure
to answer. The short answer to your question is simply "noon."
Astronomical noon is defined to be the time of day when the Sun is
highest in the sky. For a northern latitude of 40 degrees (typical of
North America) the Sun's noon position ranges from about 40+23=63
degrees South of straight-up in Late December to about 40-23=17 degrees
South of straight-up in late June. That is why it is hotter in the
summer than the winter. (Note: The Earth's spin axis is tilted by 23
degrees with respect to its circular orbit around the Sun; that is
where the 23 degrees comes in.)
TIME ZONES AND DAYLIGHT SAVING TIME
Now, Astronomical Noon is not always at 12:00 local time. In the
winter when we are on Standard Time it is within an 1/2 hour or so of
12:00 -- closer if you live near the middle of your time zone. If you
live on the western edge of your time zone, Astronomical Noon is a
little later than 12:00 because the Sun moves from East to West during
the day. If you live on the Eastern edge it is earlier. (It could
even be a few hours off if you live in parts of Alaska.)
On the other hand, during Daylight Saving Time astronomical noon is at
around 1:00 pm., because we change our clocks so we can have more
daylight in the evening when we are awake, and so the Sun does not rise
too early in the morning when we are asleep.
THE SUN'S PATH IN THE SKY
There is another astronomical effect you should know about that can
change the time that the Sun is highest in the sky. This is a little
hard to explain, but it also has to do with us watching the Sun from a
tilted perspective. The Earth spins on its axis about 366 and 1/4
times each year, but there are only 365 and 1/4 days per year. This is
because we define a day not based on the Earth's period of rotation,
but based on the average time from noon one day to noon the next.
Gradually over the course of a year the Sun appears to go 'backwards'
(West to East) around the Earth compared to the far away stars (this is
because we are really going around the Sun). Subtracting this 1 time
backwards from the 366 and 1/4 times forward, we get the typical 365
and 1/4 days per year.
So back to the point of the Sun at noon. The Sun appears to go around
the Earth in a circle from West to East once per year in an orbit that
is tilted by 23 degrees from the Equator (because the Earth's axis is
tilted). This tilt means that sometimes the Sun appears to move mostly
West-to-East, and very little North or South (like in late Dec and late
June). While at other times is appears to move a good deal North or
South (like in late March or late September), so it appears to move
So how what does this have to do with time and the Sun at noon? The
time standard is to set our clocks to 12:00 when the Sun is highest in
the sky on March 21st, since this is when it crosses the equator, also
known as vernal equinox. But as you check the Sun's position at the
same time, by your watch, on subsequent days, it appears to move West.
This is because it appears to be moving slower than average West-to-East.
Conversely, in late December the Sun looks like it is turning around
from moving South to moving back North again, known as the winter (to
people living in the northern hemisphere) solstice. It does not appear
to move either North or South, but it is moving faster than average
Now following this argument we see that right after the equinox,
in April and May, the Sun will be a little West of South at 12:00 (or
it will be highest in the sky at a little before 12:00). However, by
late June (the summer solstice) the Sun would have caught-up so it is
now highest in the sky at 12:00 again. In July and August it is now
East of South at 12:00 (or it will be highest in the sky at a little
after 12:00). And then by late September (the Autumnal Equinox) it
has fallen back to being South at 12:00 again.
There is a chart, called the Analemma (URLs below) that puts together all
these effects to show the position of the Sun at noon each day
throughout the year. You can recognize the Analemma because it looks
like a figure 8.
YET EVEN ANOTHER EFFECT
If you are even more interested in this, there are some smaller effects
because the Earth moves slightly closer and farther away from the Sun
throughout the year. (We are closest to the Sun in January and
farthest away in July). The motion toward and away from the Sun is
very small compared to the distance to the Sun, yet still large
compared to the size of the Earth. When the Earth is farther away
from the Sun, it moves slower and when it is closer it moves faster.
It is this effect that makes the Analemma not perfectly symmetrical --
it has a bigger loop on the bottom than the top. If the Earth had a
perfectly circular orbit, the Analemma would be a perfectly symmetrical
Figure 8 with the cross-over point directly above the Equator. As you
can tell from the figures (URLs above) it is not symmetrical.
I hope this leaves you lots to ponder. We really enjoyed your
question because it is so simple -- yet it can be answered simply, in
more detail or excessive detail.
HOW YOU CAN MEASURE THIS YOURSELF
One way to determine when the Sun is highest is to find the time when
shadows are shortest. Find a flagpole or other vertical object surrounded
by level ground. Every ten minutes or so, place an object, marked with the
time, at the tip of its shadow. When the shadow has stopped shrinking and
is starting to get longer again, the object closest to the flagpole will be
marked with the time at which the Sun was highest. If what we said above is
correct, the shadow should be due North. Go test this yourself and see if
we got it right!
For Ask an Astrophysicist
-- with much technical expertise from David Palmer
and help from Paul Butterworth, Karen Smale and Tess Jaffe
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