Equinox sunrise

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I visited Huntley Meadows Park to photograph sunrise on the March Equinox, 20 March 2011. I used Theodolite Pro app to record a time series of geotagged screen captures beginning at 7:12 a.m., the official time of sunrise in Washington, D.C. on March 20th. On the equinoxes, the Sun rises due east (90 degrees azimuth) and sets due west (270 degrees azimuth). Looking at photo 1 of 6, notice that the camera is facing due east (090°) and the disk of the Sun is below the tree line at 7:12 a.m.; by the time the Sun is clearly visible at 7:28 a.m. (photo 5 of 6), the Sun had moved along its path across the sky to a point slightly south of east (see Editor’s Note, below).

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I used Pro HDR app to shoot a composite image (shown above) of the scene looking eastward across the wetland. Notice there is some ghosting visible along the tree line, probably due to the fact that I was too cold to stay still for a handheld shot!

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Editor’s Note: Sunrise is defined as the time of day when the Sun’s upper limb appears above the true horizon. From my viewpoint at Huntley Meadows Park, the true horizon was obscured by the visible horizon (the tree line). Therefore, I was unable to see the Sun‘s disk on March 20th at exactly 7:12 a.m.

Planet Earth is a magnificent timepiece! The Earth rotates counterclockwise once every 24 hours. One complete rotation equals 360 degrees. The rate of the Earth’s rotation equals 15 degrees per hour:

360°/24 hr = 15°/hr or 15°/60 min, which reduces to 1°/4 min

Notice that the first five photos were taken approximately four (4) minutes apart; photo 6 of 6 was taken two (2) minutes after photo 5 of 6. Therefore, 18 minutes elapsed between the first and last photos. That means the Earth rotated 4.5 degrees during the photo shoot. Do the math:

18 min/1 x 1°/4 min = 4.5°

Now we know the Sun’s azimuth was 94.5 degrees when photo 6 of 6 was taken. No wonder it appears as though the Sun didn’t rise due east on the Equinox!

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