Home > Astronomical History, Ephemeris Program, The Earth, The Moon > Ephemeris: 05/19/2026 – The Greeks discovered that the Earth was round and determined the Moon’s distance

Ephemeris: 05/19/2026 – The Greeks discovered that the Earth was round and determined the Moon’s distance

May 19, 2026 Leave a comment Go to comments

This is Bob Moler with Ephemeris for Tuesday, May 19th. Today the Sun will be up for 14 hours and 58 minutes, setting at 9:08, and it will rise tomorrow at 6:09. The Moon, 3 days past new, will set at 1:01 tomorrow morning.

The Ancient Greeks used lunar eclipses to determine that Earth is a sphere, and worked on determining the distance to the Moon. From ancient times, the Greeks knew that an eclipse of the Moon was caused by the Earth’s shadow falling on the Moon. Since the Earth’s shadow was always circular, no matter where the Moon was in the sky during an eclipse, the Earth must be a sphere since that’s the only three-dimensional body that always casts a circular shadow. They also used the size of the Earth’s shadow to estimate the distance to the Moon. The lunar distance, on average, is 60.8 times the Earth’s radius away. The first estimates were about one third of that. Hipparchus in the 2nd century BC got much closer at 68 Earth radii.

The astronomical event times given in this blog are for the Traverse City/Interlochen area of Michigan (Lat 44.7° N, Long 85.7° W; EDT, UT – 4 hours) unless stated otherwise. Times will be different for other locations.

Addendum

Partial Lunar Eclipse showing arc of the Earth's shadow
Partial Lunar Eclipse showing circular arc of the Earth’s shadow. Taken by me from the Veen Observatory of the Grand Rapids Amateur Astronomical Association at 04:15 UT August 17, 1970.
The partially eclipsed Moon setting through a thin cloud and the neighbor kid’s swing set.
The partially eclipsed Moon setting through a thin cloud and the neighbor kid’s swing set at 7:09 AM EDT April 4, 2015. Sunrise that morning was at 7:18 AM. Taken with a smartphone through 10X50 binoculars. If the Earth were pizza shaped, it would cast a shadow like the cloud in the image.
Earth and Moon size and distance to scale.
Earth and Moon size and distance to scale. Modified from the original GIF animation by the author.

Categories: Astronomical History, Ephemeris Program, The Earth, The Moon Tags:
  1. No comments yet.
  1. No trackbacks yet.

Leave a comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.