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We think of the moon as orbiting the earth, following a spiraling trajectory as the earth itself orbits the sun. But this is wrong. Not only is the moon's orbit NOT a spiral... there's an argument that the moon actually orbits the sun, not the earth! The moon's trajectory is more like a 12-sided polygon with curved corners than it is a spiral or even a wiggly line.

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Pressure *can* melt ice - like, you only need 500 times atmospheric pressure to melt ice down to negative four degrees celsius. If you have 1000 times atmospheric pressure (like at the bottom of the Mariana trench), then you'll melt ice down to around negative nine celsius. But that's for fresh water. For salty sea water, things are different.

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Solar eclipses don't just happen here on earth - moons of other planets also pass between those planets and the sun, resulting in various types of solar eclipses on Mars, Jupiter, Saturn, Uranus, Neptune, and even non-planets like Pluto, Eris and various asteroids. So, where are the best eclipses in the solar system? For that, we need a tier list.

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The moon orbits the earth once per month, which means the moon is on the sun side of the earth every month. So... "why aren't there eclipses every month?" is a question we will answer in this video!

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If you’re in scorching heat, or when your body is working hard and you’ve got hot, hot sweat all over, sticky and stifling - does wiping off the sweat help you cool off? Or is it better to leave it on?

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**********************************
REFERENCES

Hyperphysics:
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/sweat.html

Engineering Toolbox Mollier Diagram:
http://www.engineeringtoolbox.com/psychrometric-chart-mollier-d_27.html

Sweat Info
http://www.anaesthesiamcq.com/FluidBook/fl3_3.php

Other articles:
http://www.slate.com/articles/health_and_science/explainer/2012/06/should_you_wipe_away_your_sweat_or_does_that_keep_you_from_cooling_down_.html
http://onlinelibrary.wiley.com/doi/10.1111/j.1748-1716.2012.02452.x/abstract
http://lifehacker.com/5921036/dont-wipe-your-sweat-off-your-brow-itll-cool-you-down-faster
http://www.realclearscience.com/2012/06/28/does_wiping_sweat_prevent_you_from_cooling_off_247729.html

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CALCULATIONS

Typical adult human body surface area ~ 1.5-2 m^2
https://en.wikipedia.org/wiki/Body_surface_area

Evaporation rate at 25°C and 50% humidity, slight air movement (v~.5m/s) = .35kg/m^2/hr
http://www.engineeringtoolbox.com/evaporation-water-surface-d_690.html

So in these conditions, a sweat-covered human can expect to evaporate ~.5-.75 L of water in an hour (For higher humidity (60-70%) it goes to ~.37-.5 L of water/hr). That amounts to ~0.25-0.35mm of sweat (covering the whole body) evaporated in an hour, or 6 micrometers every minute.

Water has latent heat of 2,270 kJ/kg (http://www.engineeringtoolbox.com/water-thermal-properties-d_162.html), so in an hour a human can lose ~1100-1700 kJ of energy. (2270/4.1868 ~ 542 Cal)

BUT that assumes all of the energy came from the person. If some proportion of it came from the air (~1/3-1/2?) then the person is only cooled down partially.

Mass of a human ~ 60-80kg (https://en.wikipedia.org/wiki/Body_weight), assuming ~specific heat of water, ie 4 kJ/kg/K, could decrease temp by ~4.5-5°C.

Energy used in moderate-hard exercise is ~20-30 kJ/kg/30 min, or ~40-60kJ/kg/h (http://www.weightloss.com.au/weight-loss/weight-loss-tools/exercise-energy-charts.html). Let’s say 50kJ/kg/h, which for average human amounts to 3000-4000 kJ/hr

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A lot of people took pictures of the recent solar eclipse in North America and got photos where there’s a ghostly image of the eclipse floating in the sky nowhere near where the sun is!

REFERENCES:
Lens flare prediction based on measurements with real-time visualization https://doi.org/10.1007/s00371-018-1552-4
Physically-Based Real-Time Lens Flare Rendering http://doi.acm.org/10.1145/1964921.1965003
From the Series of Articles on Lens Names: Tessar, by H. H. Nasse. Carl Zeiss Camera Lens Division March 2011

https://www.cambridgeincolour.com/tutorials/lens-flare.htm
https://www.toolfarm.com/tutorial/in-depth-lens-flares-for-video/
https://petapixel.com/what-is-lens-flare/
https://www.maxon.net/en/red-giant/vfx-suite/real-lens-flares

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The first video we created together with our friends at xkcd is out now! You can find it here: http://youtube.com/@xkcd_whatif