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How to Calculate the Date by the Sun

The Earth's tilt and rotation cause the sun to rise and set at a different time each day. You can can calculate the day of the year by comparing the time of sunrise and sunset to known values for your latitude and longitude. Sunrise is defined as the time at which just over half the sun has risen above the horizon, and sunset is when just over half the sun has disappeared. Many days have a distinct pair of values, but since some days vary only by seconds, you may need to observe the sky for several days to calculate the exact date.

Things You'll Need

Watch
Sunrise/sunset chart
Pen
Paper

Instructions

- 1
  Record the exact time of sunrise on a piece of paper. It is important that you be accurate to the minute, so make sure that your watch is synchronized with the official time for your region.
- 2
  Record the time of sunset in the same manner as the time of sunrise.
- 3
  Find an identical pair of times on the sunrise/sunset chart. Many charts will have multiple columns with the time of sunrise appearing first in each column and the time of sunset appearing next to it. Note that these values are often expressed in military time.
- 4
  Look at the top of the column in which your values appear to find the month of the year.
- 5
  Look across the row in which your values appear to find the day of the month.

Astronomy