Question: why don't we see shadows from satellites

Iain Bethune answered on 16 Jun 2015:

Satellites are too small and far away to cast a shadow. Low-earth satellites (including the International Space Station) are a few hundred miles above the surface of the earth. Geostationary satellites (like TV and communications satellites) are about 22,000 miles up.

To cast a shadow something needs to be big enough to completely block out the light source. Try holding up a penny in front of a light bulb. If you move a bit of paper close up to the penny you will see a shadow. This is because the at some points on the paper, the penny completely blocks out the light bulb. These bits are in shadow. You might also see some partial shadows at the edge, where only part of the light bulb can reach the paper. If you move the paper, the penny and the bulb further apart, the shadow will disappear, since it is too small (relatively) to block out the light of the bulb from any point on the paper. Try it out and let me know how you get on!

In the same way satellites are too small to cast any shadow. The moon is big enough though and when you get a solar eclipse, then the part of the earth in the moon’s shadow is called the ‘umbra’ – this is where you get a total eclipse. The partial shadow is called the ‘penumbra’, and there you see a partial eclipse. I took some photos of the eclipse earlier this year – here’s one:

Interesting fact – when something small does pass in front of a star like the (say a small planet), then a tiny fraction of the light from the star is blocked (<1%). Sensitive telescopes like Kepler can detect this and it's one of the ways we use to detect exo-planets – planets around other stars. This is called the 'Transit Method', because the the planet has to transit in front of the star in order for us to detect it!