The New Zealand Herald has an article today about a cool and very popular image of the Moon positioned perfectly within a radio satellite, produced by astrophotographer Chris Pegman: Supermoon image goes into media orbit

Image by Chris Pegman
Image by Chris Pegman

The article talks about how there has been debate online about whether or not this could be taken without resorting to Photoshop. It concludes that “the verdict was that it might be, but it would require an incredible amount of planning” but this isn’t strictly correct.

The apparent rotation of the Moon changes as it travels through the sky. When it rises, it will appear to be “on its side” relative to when it is at its zenith, and when it sets it will have rotated further still.

This is most obvious with a crescent Moon. Depending on if it’s waxing or waning, the Moon will rise with the crescent facing either down or up, then when it’s at its zenith the crescent will be facing sideways, and as it sets it will have rotated around further. Of course, the lit side of the Moon always faces the Sun. It’s the fact that the Earth rotates beneath us that makes it look like the Moon is rotating as it travels across the sky.

Here’s an example of this which I took with my phone in July, showing a waning crescent Moon shortly before sunset:

We can see from the lunar maria (the dark areas) that the Moon in Chris Pegman’s picture is rotated how it would be if (when viewed from the southern hemisphere) it were near its peak, not near the horizon, so his picture couldn’t be produced without artificial manipulation.

Mark Gee is a fantastic astrophotographer from Wellington. In October he captured a time lapse of a full moon rising, in which you can clearly see that angle of the Moon is not the same as in Chris Pegman’s image when it rises: Supermoon rises over New Zealand timelapse.

There’s a Twitter account called Fake Astropix, which tweets fake astronomical images with the reasons why they are recognised as fake (well, as much as can be given within a tweet).

I find these reasons can be very educational and thought provoking. For example, it’s impossible to take a photo from Earth where the Sun and Moon don’t appear to be roughly the same apparent size. Also, the full moon can’t appear next to the Sun in the sky (remember the lit side faces the Sun). So “debunking” these fake astronomical images can be a good educational exercise that makes you think a bit more carefully about how things work in our solar system.

Do you have any fake astronomical images that you can share, along with the reason why you can tell it must have been faked?

Have you seen any astronomical images that you think might be fake but you’re not sure? Share them here and let’s investigate, and see if we can learn something.

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