Is the Sun directly overhead?

Robert Bigelow

In my March missive on Daylight Saving Time, I mentioned a misconception that is held by some students and possibly some adults as well. A more common misconception concerns the apparent height of the Sun in the sky. When I ask students the question, “When is the Sun directly overhead in Utah?,” most students answer, “every day at noon.” The correct answer is NEVER.

For those of us that live north of the Tropic of Cancer, the Sun appears highest in the sky every day when it is due south. Its height on a particular day is determined by Earth’s orientation to the Sun. To quote from that previous post:

“Earth’s rotational axis is tilted by about 23.4º and points in a nearly constant direction as Earth circles the Sun. This is evidenced by the northern axis pointing toward Polaris, the North Star. While the axis continues to point in the same direction, it’s orientation to the Sun changes. Back on December 21, Earth was at the place in its orbit where the northern axis leans most away from the Sun.” This is illustrated in the diagram below.

WinterAxis2

The Sun has a diameter 109 times that of Earth. To correctly show how light from the sun strikes Earth at 40 degrees N latitude, the circle representing the Sun in the diagram has been placed too high with respect to Earth’s center.

In the diagram, light from the Sun is shown striking Earth at 40 degrees N latitude (Salt Lake City, Utah). An observer standing upright at 40 degrees N latitude would be on the line perpendicular to the horizon line. The angle between the horizon line and the line from the Sun is the angle that the Sun’s rays strike the surface of Earth. It is the angle that an observer in Salt Lake City would measure when the Sun is due south. On December 21, the angle between the horizon and the Sun is about 26 degrees, so we see the sun low in the sky at noon. (To see this better, turn the diagram so the horizon line is horizontal).

Back to the March post: “As Earth continues to move around the Sun, the angle between the axis and the Sun decreases.   . . .  until June 21, when Earth reaches the place in its orbit where the northern axis leans most toward the Sun.”

SummerAxis2

On this date we see the Sun high in the sky, but NOT directly overhead. For Salt Lake City, the Sun is about 73 degrees above the horizon. This is actually the highest we ever see the Sun.

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3 thoughts on “Is the Sun directly overhead?

  1. I’ve found your insight very interesting.
    Your knowledge of “Dark Matter” and how do a “size comparison” of the sun is also facinating. You are my current expert in Astrophysics.
    It’s like you are the North Star, stable, reliable, fixed in your location, and the basis of knowing I am on the right path, if I follow your lead. So far so good.
    Your articles are very well written. I’m so proud of you and the success you’ve had making an impact on the next generation.
    An old friend,
    Marilyn

  2. The recent earthquake in Japan last March 2011 shifted the earth’s rotational axis by another 6.5 inches so this throws off the current figures stated above.

  3. Great explanation! I didn’t know this. All I know is that sun is directly overhead in Earth during 12nn. Good thing that I read this article. I’m going to share this to my kids. Their teachers will be surprised for sure that they already know this.

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