Like a big pizza pie

About a week ago, I was running a tour at Sydney Observatory and had an awesomely engaged kid on my tour who entertained everyone with stories about how, back in the dinosaur age, the Moon was so big in the sky and it was visible over the side of a cliff and it was Halloween… I’m assuming this was a reference to some documentary or movie about dinosaurs, but it did get me thinking nonetheless!

Firstly: just how big was the Moon back in the dinosaur era? We’ve measured now that it’s moving away from us at about 4 cm/year, and although that’s not been constant over time (it’s actually somewhat faster than expected) it’s a good place to start. I also often remind people that, due to this lunar retreat away from Earth, it means one day we won’t have total solar eclipses anymore because the Moon will be so far away it never completely blocks the Sun. I’d never done the calculation for just how long that would take, so it made sense to have a go at this while I was at it.

Here is a very basic infographic I put together showing the angular size (size on the sky) of both the Moon and the Sun, as well as how it changes as the Moon gets closer and further away on its non-circular orbit. I’ve also answered the two questions above at the bottom of the graphic, so check it out!*

*This was done using Pythagoras’ theorem, along with the following Google-acquired data on the Moon radius, Sun radius, Moon closest/average/furthest current distance, the Sun’s distance and the start/end of the dinosaur age (if you’re curious or want to correct my values!):

Moon radius 1737 km
Sun radius 696342 km
Moon distance (closest) 356500 km
Moon distance (average) 370300 km
Moon distance (furthest)
 406700 km
Sun distance 149.6 million km
Triassic start 250 million years ago
Cretaceous end 65 million years ago





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