Dr. Aaron Titus | Department of Physics, High Point University PHY1050      Astronomy of Stars, Galaxies, and the Cosmos home | WebAssign | textbook | course calendar

## motion of Sun

Sun's path across the sky depends on the day of the year and the latitude at which you live.

The latitude of the Triad is about 36° N. We will focus on the path of Sun as viewed at our location.

You have heard that Sun rises in the East and sets in the West. That's not quite as precise as we like to be in science. Let's be scientists and figure out where Sun rises and sets depending on the day of the year.

The following images show Sun rising on the 21st of each month, starting with September.

There are two days out of the year when Sun rises due east. What days are these?

Which day out of the year does Sun rise the furthest northeast?

Which day out of the year does Sun rise the furthest southeast?

To see Sun rise at different locations, hold the up or down arrow key on your computer to quickly scroll the page up and then down. You'll see Sun move back and forth across the horizon.

Note that the dates for the solstices and equinoxes are not exactly the 21st. It varies slightly depending on the year. You can see a list of dates for the solstices and equinoxes at the USNO web site.

## Local Noon

Local noon is the time when Sun crosses the Meridian, that imaginary line that runs across your sky from the southern horizon through the point directly overhead (called your zenith) and to the northern horizon.

The altitude is the angle from your horizon to an object in the sky. Imagine altitude in the following way. If you point toward your horizon with your arm and then raise your arm directly upward to a star, then altitude is the angle through which you moved your arm.

The altitude of Sun at local noon depends on the time of year. Furthermore, the time on your clock when local noon occurs varies depending on the time of year.

For example, compare the altitudes of Sun in the following images. (Note that the rings you see are due to the camera lens and are not objects in the sky. Sun is indicated by the yellow dot.)

On the winter solstice, Dec 21, Sun rises SE, takes a very low path through the sky and crosses the Meridian at its lowest altitude, only 30° above the horizon.

On the vernal equinox, Mar 21, Sun rises due east, crosses the Meridian at an altitude of 53° above the horizon, and sets due west.

Sun is at its highest altitude at local noon on the summer solstice, approximately June 21. For this reason, this day is the longest day of the year. Sun rises NE, takes a high path across the sky and crosses the Meridian at its highest altitude, 77° above the horizon, and then sets NW.

On what day will Sun be directly overhead of us, here in the Triad?

Our latitude is 36 degrees N. So, if we look directly overhead we will see a point on the celestial sphere that is at a declination of 36 degrees N. The highest declination of Sun is 23 degrees N, and this is the summer solstice. The difference in these angles is 13 degrees. Thus, Sun is never directly overhead of us.

Those who live at a latitude of 23 degrees N will see Sun directly overhead at local noon on one day--the summer solstice.

Those who live at a latitude of 23 degrees S will see Sun directly overhead at local noon on one day--the winter solstice.

Those who live at the equator will see Sun directly overhead at local noon on two days--the vernal (Mar 21) and autumnal equinoxes (Sept 21).

Those who live at latitudes between 23.5° N and 23.5° S will see Sun directly overhead on two days of the year. The exact days depend on their latitude.

You should know how the path of Sun across the sky varies throughout the year, including where it sets and where it rises and its altitude when it crosses the Meridian. Also, know how this varies with the latitude where you live.

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