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

Comments (31)

  1. I have seen movies get the motions wrong. The movie Rio, set in Rio, had a sunrise going the wrong way. A few years ago, I saw a commercial (forget what it was for) that was set in the U.S. but had a sunrise going the wrong way. I’m pretty sure the filmed a sunset and ran it backwards thinking now one could tell!

  2. Chris

    So is there an iridium flare in this video?

  3. Yes, the motion of the sun and stars is odd for people in northern hemisphere. In this cases I think it is useful a software like Stellarium (http://www.stellarium.org/), you can set your location to anywhere in southern hemisphere and change the time into Stellarium to see how the stars, the moon and the sun move across the sky. It’s also worth to try a location at the ecuator.

  4. Musical Lottie

    Your illustration of trees passing a car really threw me until I remembered that you’re not in the UK. Then it made sense!

    I shall look out for sunrises/sunsets in films and TV programmes now; I’d never thought of the way the sun moves in the different hemispheres before. Fascinating! :)

  5. Other Paul

    So, to fake a sunrise, instead of filming a sunset and running it backwards, a film crew could just pop south and film a sunset going forwards. They’d still get the photogenics. And it would look odd only to southern hemispherers who are – presumably – already used to northern hemispherism.

  6. alfaniner

    Question: If they flopped the film, would anyone be able to tell the difference?

  7. SkyGazer

    Always funny how people say “I can only guess how dark the skies must be there”.
    I remember the really dark skies of my youth on Formentera. And those clear nights were bright as hell! In those days (mid 60s to begin 70s) there was one bar (run by Amador) that had a (one) electric bulb and a fridge and a ginormous generator. Homes were lit with oillamps and candles. When there was no moon there was enough light to see were you were going.
    And that´s just starlight.

  8. It’s the Coriolis force, isn’t it, that causes cars to pass each other on the other side in the Southern Hemisphere, right? And if you try to drive on the Equator, you always collide with the cars coming the other way.

    Laws of physics. You just can’t beat ‘em.

    Lovely video, by the way.

  9. “I knew it was filmed in the southern hemisphere. Can you guess how?”

    Pleiades are upside down. :D

  10. StarkRG

    “I knew it was filmed in the southern hemisphere. Can you guess how?”

    Guy’s name was Murray, such an Aussie name… :P

  11. Bob

    I once had a creative writing teacher whose editor pointed out that her character was watching sunrise and sunset from the same window. Oops.

  12. Navneeth

    Gonçalo Aguiar,
    I assumed it was the constellations as well. :)

  13. ASDCR

    how can the camera pick up stars and the sun in the same scene??

  14. Doug Jones

    The sunrise at 5:06 was either filmed in the northern hemisphere, or flipped L-R.

  15. Mario

    I guess you cleverly deduced the hemisphere of the video’s origin by reading its caption on vimeo: “Time-Lapse short produced at Lake Eyre in central Australia as part of the SALT project”


  16. Peter Tibbles

    As far as I can see, everything is the right way up and moving in the correct direction, but then, I live in Melbourne.

  17. Pete Jackson

    @11Peter: Yes, you need to stand on your head to see this the proper way. :-)

  18. For Lake Eyre there is an unusually large amount of cloud present.

  19. Tara Li

    Yeah – I’m missing why this was titled “Iridium” as well.

  20. jjmcgaffey

    8 Skygazer – I remember that from rural Virginia in the US. We had a permanent campsite, and while there were (dim) streetlights on the main roads we were well back in the woods. I could easily go the quarter mile to the bathrooms after dark purely by starlight – though where the tree-shadows crossed the road was really dark! There’s not enough light from starlight to scatter the way sunlight does, so starlight shadows (and to a slightly lesser extent, moonlight shadows) are much darker than shallow shadows like trees in sunlight (caves etc can get that dark, since the sunlight never gets in).

  21. Ed

    I’m brazilian and every time I travel to the northern hemisphere I have a lot of trouble recognizing stars and constellations. And when I (jokingly) tell people that the moon is upside down nobody ever seems to understand what I’m saying…

  22. CR

    @ ASDCR (14)… that’s not the sun visible with the stars, it’s the moon. (And I have to admit that in some of those night shots, I was tricked by the play of moonlight on the salt flats… I thought I was looking at a verrrrrrry still water surface!)

    I loved the shots in the video of the galaxy turning overhead… the particular one with the bright blinking light had me wondering which star that was… Sirius? I didn’t pay attention to the surrounding constellations to properly get it. Still, very neat.

    As for the video’s title, might it have something to do with the music accompanying it? The song is titled “Iridium.”

  23. Ronald

    The wrong way?

    Nothing wrong with that!

    Ronald McCoy

    Melbourne, Australia

    Seriously, good posting. I really enjoyed it.

  24. Naomi

    Oh man, that was the most disconcerting thing when I did a semester in the States. I could deal with the food, I could deal with cars on the wrong side of the road – but then the moon rose from the wrong direction and upside down and my brain just went WHAT.

  25. bassmanpete

    Lake Eyre, a dry salt bed in central Australia.

    Normally yes, but it’s been wet for over a year now although drying up rapidly at the moment.

    And I have to admit that in some of those night shots, I was tricked by the play of moonlight on the salt flats… I thought I was looking at a verrrrrrry still water surface!

    You were, that is water. Notice the water birds in the day time shots. Millions of them went there to breed once the waters arrived.

  26. db26

    Brilliant videography, unfortunately the music was lacking. The flute didn’t sit well with me at all. Would have been spectacular set to Staralfur by Sigur Ros.

  27. Nigel Depledge

    The BA said:

    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.

    That’s not northern hemisphere bias, that’s USA-centrism. Other countries in the northern hemisphere drive on the correct (i.e. left) side of the road – it’s not just Australia and New Zealand.

  28. jojodancingbear

    Really OT, but yall check out this comic about JWST


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