Earth Is Tilted at 23.5 Degrees

by on October 16, 2007

Q: How do we really know that the earth is tilted at 23.5 degrees?

A: First, let’s clarify that the tilt mentioned in your question is in relation to our sun. Now let’s think about observations that anyone in our modern world can make of the sun. For one thing, the sun does not rise at exactly the same time or in exactly the same place from one day to the next. Likewise, the sun is not in the same place in the sky at any time today as it will be during the same time tomorrow. We could use photographs of the sun in the sky at a certain time of day for several days to prove this.

Also, consider that the amount of time the sun is in the sky is different at each degree of latitude. For instance, the amount of time that the sun is up on a given day in Alaska is different than during the same day in Texas. An exception to this rule occurs during periods of equinox. Ancient and modern scientists alike agree that the best explanation for all of this is that the earth is tilted in relation to the sun. But, how can we calculate the amount of the tilt?

According to a Middle School Astronomy Activity written by Judy Young and Bill Randolph, only basic math skills are required to make the calculation. Observe where the sun is in relation to the horizon from a specific spot on the ground at a certain time of day during an equinox, and then again during a solstice from that spot at the same time of day. Subtract the lesser result from the greater and you should arrive at about 23.5 degrees. To increase the accuracy of this exercise, you could take measurements of many equinoxes and solstices. Interestingly, the earth’s tilt changes slightly over a period of about 41,000 years. The predicted minimum is 22.1 degrees and the supposed maximum is 24.5 degrees. A page on the NOAA website has an interesting explanation of this cycle of change.

2 Responses to “Earth Is Tilted at 23.5 Degrees”

    ya what he said!


    I lost a bet on 23.5 because I hear that the earth’s tilt was between 22.1 and 24.5 over the last 42,000 years. The 1,000 year difference should not matter however because the bet’s tolerance for error was + or – 0.00000000!

    Thank you