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Dr. Bozidar It says here at summer solstice is actually valid any time after
Mitrovic: the vernal equinox and before autumnal equinox. Here is directly
east. After the one vernal equinox, the Sun rises directly east and
sets directly west.
The Sun is also west of the equator. But after the vernal equinox,
the Sun is rising north of east and setting north of west more and
more every day after the vernal equinox up until so-called summer
solstice when the Sun is highest in the sky.
On the day of summer solstice, we have something like this. Here is
the Earth. I'm going to draw a little bigger diagram because I want
to point out a few more things. Here is, say, the North Pole, the
South Pole, the Earth's rotational axis. Here's the sunlight.
This is the direction perpendicular to the ecliptic. This angle
now, on the day of summer solstice, is 23.5 degrees. Of course,
this is equatorial plane.
Now the sunset line would be this line here. This would be the
illuminated half of the Earth. This side here would be in dark. Now
what is the latitude of this point here that is on the sunset line
which divides daylight from darkness?
If you know that this angle here is 90 degrees and that this angle
here is 23.5 degrees, how big is this angle here?
Man: 66.5.
Dr. Mitrovic: 66.5 degrees.
[pause]
Dr. Mitrovic: The circle on Earth with a latitude of 66.5 degrees is known as the
Arctic Circle.
Note that on that day, on the day of the summer solstice, all the
points on the surface of the Earth with a latitude greater than
66.5 degrees north, that is they are above the Arctic Circle, they
are always illuminated by sunlight.
That is the day lasts 24 hours. On the other half, here is the
Antarctic Circle with a latitude of 66.5 degrees south. The
latitude is 66.5 degrees north. At the latitudes bigger than 66.5
degrees south on the day of summer solstice, you can't see the Sun.
You get 24 hours of darkness.
Above the Arctic Circle, you get 24 hours of daylight. Below the
Antarctic Circle, you get 24 hours of darkness. Of course, as you
move south from the Arctic Circle, you get more and more hours of
darkness but, nevertheless, the days are longer than nights.
The same in the Southern Hemisphere. As you move towards the
equator at latitudes less than 66.5 degrees south, you get more and
more daylight as you approach the equatorial plane.
Now let me just show you one spectacular photograph of what you see
over a period of 24 hours on the day of summer solstice while being
on the Arctic Circle. This is the day of summer solstice. The Sun
is always above the horizon even at midnight.
Looking north, you can see that the Sun just grazes the horizon. It
doesn't fall below the horizon. Then after the midnight, it starts
coming up. Here it is at 6:00 AM. It's highest at noon. Then it
starts coming down, coming down but it's always above the horizon.
The daylight is 24 hours long.
After the vernal equinox, the Sun moves above the celestial equator
by greater and greater amount. The maximum elevation above the
celestial equator is achieved on the day of summer solstice. Then
it starts moving up.
Whenever you have an object actually moving very slowly like if you
hear the pendulum that is swinging back and forth, at extreme
points, it stops and reverses direction. The speed is reduced to
zero. Then it's picking up again. It looks as it's moving high and
higher that the speed with which the Sun is rising while moving
along the ecliptic is reduced.
It looks as if the Sun stopped. Hence, the name solstice.
A similar effect will be on December 22nd when the Sun stops its
motion below the celestial equator.