Time lapse: IRIDIUM

By Phil Plait | October 23, 2011 7:00 am

[Note: ROSAT fell to Earth last night; see this post for details and links to more info.]

Time lapse videos can be breathtaking, lovely, and a joy to watch… but they can also show you something you may not have thought about before. Before I even read the caption for Murray Fredericks’ video called "IRIDIUM", I knew it was filmed in the southern hemisphere. Can you guess how?

[Make sure to watch it in HD, and make it full screen.]

If you live in the northern hemisphere — and odds are very good that you do — then you may have noticed the motion of the Sun and stars looked a bit odd. For example, as you watch the Sun set at the beginning of the video, it does so at an angle moving from the upper right to the lower left. The stars do too. When they rise, they move from the lower right to the upper left.

To me that’s backwards!

If I face west, looking toward the sunset, I see the Sun moving from my upper left and moving to the lower right. As I explain in an earlier post, that’s because when you head south, below the equator, your directions flip. Facing north in the northern hemisphere, west is to my left. Facing south in the southern hemisphere, west is to my right. As I wrote before:

Think of it this way: imagine you’re in a car, driving on a road going through a forest. If you face left, looking out through the driver’s window, the trees appear to pass you from your right (the front of the car) to your left (the back of the car). Now turn around and look out the passenger window: trees move from your left to your right! Directions reverse because you’re facing the other way. The same is true for the sky, so while rising stars appear to move counterclockwise when you look to the north, they appear to move clockwise when you face the south.

So when I see the motions of stars in videos like the one above, I get a little disoriented. I was in Australia a few years ago, and it totally threw me off. The Moon’s crescent faced the wrong way! Orion rose upside-down! My shadow pointed the wrong way! People drove their cars from the passenger side!

Hmph. It occurs to me that in the quote above, I assume the driver’s side of the car is on the left. Between writing that, and still feeling weird watching videos like IRIDIUM, I guess I still have lingering traces of northern hemisphere bias.

Another thing about this: in a lot of movies and TV shows, they use sped-up footage of the Sun rising (usually on a beach or over a city) to denote a jump forward in the plot line to morning, a way to show that we’ve moved ahead a day in the story. I used to wonder why, in many of those scenes, they showed the Sun moving backwards, from right to left, as it rose. I found out that they tend to use sunsets for those clips, then run the video backwards! Why?

It’s because it’s easier to set up a camera in the late afternoon and shoot a sunset; you can see where the Sun is headed, and aim the camera there with time to spare. It’s harder to know exactly where the Sun will rise if you’re outside and haven’t done the calculations and measurements, so getting a sunset is simpler and faster. Also, sunsets tend to be redder than sunrises due to more junk in the air during the day, so it’s more photogenic. And of course it’s also just easier to get a film crew to work a little late to get a sunset on camera than it is to roust them out of bed hours before sunrise in the early a.m.

Astronomy! It affects us all, even Hollywood types.

Which brings us back to the video. It was created as part of a movie called "SALT", about Fredericks’ annual five week pilgrimage to Lake Eyre, a dry salt bed in central Australia. It’s beautiful, and shows what you can see when there’s little or no light pollution. I can only guess how dark the skies must be there, and how incredible it must be to watch the starry vault slide by overhead…

Tip o’ the lens cap to Tom Lowe on Google+.

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

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