I’ve seen lots of morning news shows that begin with a video of a “sunrise.” But oftentimes it’s actually a video of a sunset running in reverse. Why? Because it’s easier. The videographer doesn’t need to predict where to point the camera so the rising Sun is in-frame, doesn’t have to wake up early to catch the event, and figures nobody can tell the difference anyway. But astronomers can tell the difference — many fake sunrise videos retain a geometric clue to the deception.
The time lapse video above shows a typical sunset in the West. North is to the right, South is to the left. Notice how the Sun drifts northward as it sets. On this date (May 1st, 2013) in Arizona, the setting Sun moves at a 60° angle relative to the horizon. The exact angle depends on latitude and season, but there’s always a northward drift to that motion.
If you simply reverse this video, the Sun will appear to drift southward as it “rises.” During a sunrise, which occurs in the East, the Sun’s drift is indeed southward, but the motion is from left to right. If in addition to running the video backwards, the videographer also flipped the video horizontally (left and right), then it would look like a “real” sunrise.
I should point out that the preceding descriptions of sunrise and sunset applies to observers in the northern hemisphere. South of the equator the Sun also rises in the East and sets in the West, but its north- and southward drifts are reversed.
If you were surprised that the Sun doesn’t set straight down, perpendicular to the horizon, then you haven’t been watching enough sunsets. The Sun almost always moves in a slanting path as it sets (and rises). The only time it goes straight down (or up) is at an Equinox, and then only if you’re observing from exactly on the equator.
Next time you tune in to your favorite morning news show, watch for a fake sunrise. If they took the lazy route and used a sunset running in reverse, you’ll know what to look for. If they cheated, call them on it. Or send them to Sky Lights.
Time lapse notes: This sequence was shot at f/32 through my Zuiko 200 mm telephoto lens. That lens doesn’t “talk” to my Canon EOS 20D, but experience suggested aperture priority was the best mode. The Canon started with a shutter speed of 1/3200 sec and finished with 1/200 sec. Thus, each frame had to be manually adjusted to reproduce the natural darkening of the sky at sunset. Frames were shot at 30 second intervals, so the animation is running about 60 times faster than real life. It was windy that day, and there’s a haze of dust in the air. What appear to be cacti moving around on that hill are the leaves of a jojoba bush in the foreground blowing in the wind.
Next Week in Sky Lights ⇒ How Telescopes Work – Part 2