If we want to reach for the stars, we have to start in our own backyard.
By looking at the sky alone, we can see the apparent, angular sizes of the Sun and Moon.
But when the Sun, Earth, and Moon all align to produce a total eclipse, it teach us how far astronomical distances truly are.
During a lunar eclipse, the Earth’s shadow falls on the Moon; showing that the Earth is almost perfectly round.
Local measurements of Earth’s curvature, combined with roundness, determine Earth is a sphere with a radius of ~6,400 km.
Geometry teaches us what the angular size of Earth’s shadow is relative to the Moon.
The recent, apogee lunar eclipse of July 27th gave a shadow-size of 260% the Moon’s apparent diameter.
This corresponds to an Earth-Moon distance that’s around 64 times the radius of Earth: 400,000 km at apogee.
It also teaches us the distance of Earth’s shadow-cone: 220 Earth radii: 1,400,000 km.
From these figures and observations, we can derive the Earth-Sun distance as well, now known to be 23,000 Earth radii: 150,000,000 km.
The Earth-Sun distance can then be used in conjunction with parallax or relative brightness measurements.
Solely from measuring the Earth, Moon, and Sun, humans can determine how far away the stars are.
Mostly Mute Monday tells the astronomical story of an object, class, event or phenomenon in visuals, images, and no more than 200 words. Talk less; smile more.Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.