Dave Wallace's Adventures in Astronomy

06/30/2004

I was looking over some vacation photos and came across one I particularly liked. And I suddenly realized that the photo told an astronomical story: the picture included an accurate time-stamp. After a bit of analysis using some computer tricks (to get the picture to reveal nearly-hidden detail in the sky) and a planetarium program to compare the image against star and planet alignments, I was able to determine to within a minute or so exactly when the photograph was taken.

Here's the picture:

There were some things that I knew before I started:
1. The location where the picture was taken was known: the Royal Lahaina Resort at Kaanapali, Maui, HI (22d55m54s x 156d41m33s). I believe it would be possible to determine the geographic coordinates from the photo, but only if a lot more astronomical objects were visible and identifiable.
2. The year (2002) and month (December) were known, though the exact time and date were not. (Actually, the approximate time of day was also known but need not have been because it can be determined from the information in the photograph alone.)
3. The planet in the picture was known (Saturn). This turns out to be key to identifying the two fixed stars in the photo, given the year and month.

Analysis:
1. The direction can be determined (west) because from that particular location the only way to photograph the moon over water and get an island (which happens to be Moloka`i) on the horizon would be to face west. (As a further correlation, the orientation of the astronomical objects with respect to the horizon would not be consistent with facing east from a point on the Earth opposite Hawaii -- the rising full moon would be closer to the eastern horizon than Saturn with the astronomical objects arranged as they are).
2. The direction (and therefore moonset as opposed to moonrise) in combination with the moon being about full fixes the time as early morning.
3. The field of view (about 28 degrees) can be determined by the apparent distance between the two fixed stars in the photo. This works out to a 72 mm focal length lens, give or take about 5 mm. This is consistent with the equipment I carried on the trip (a 28 - 200 mm zoom on a 35 mm SLR).
4. Everything else falls out by analyzing the position of the astronomical objects with respect to each other and to the horizon.
   a. If the month was not known, the position of the moon with respect to Saturn would have determined it.
   b. If the year was also uncertain, the position of Saturn with respect to the fixed stars would have determined it (but identifying the RIGHT two fixed stars would have been a real challenge!); in this case, knowing the year and month and the fact that the planet was Saturn identifies the two stars (Zeta Tauri on the left and Beta Tauri on the right).
   c. The position of the moon with respect to the fixed stars in December 2002 can only occur on December 19th and within an hour one way or the other of 1600 UTC (or 0600 HST).
   d. The height of the moon above the horizon, once the field of view is determined, further refines the time to within a minute or so: 06:02 HST.

And this is a star chart, courtesy of "Cartes du Ciel", scaled to the same field of view and drawn for 16:02 UTC on 12/19/2002, with an observer at coordinates of 20d55m54s N x 156d41m33s W: