Earth’s dark umbral shadow projects into space for almost exactly 1 million miles (1.6 million kilometers). Tapering like a chopstick, it easily sweeps past the Moon at one-quarter that distance, though its width is down to just 5,400 miles (8,700km) at that point. We’ll next see it envelop the Full Moon on April 15 next year, like a reminder from the IRS. Any tax-evading lunar colonists would witness Earth totally blocking the Sun.
If astronauts ever venture more than a million miles from here, in any direction, they’ll see the Sun appear larger than Earth — meaning our planet can no longer fully block it. Eclipses cease to exist. Beyond that point, there’s no refuge from the fierce sunlight, our world being no more than a large black dot on the solar disk.
From the surface of Mars, the last transit of Earth across the Sun took place May 11, 1984. We never had a spacecraft there to photograph such a transit; even if they could have performed solar photography, the Viking landers stopped making contact with us in 1982.
The next Earth transit will happen November 10, 2084. Our world casts its shadow on Mars at intervals of 100.5, 79, 25.5, and 79 years, always in May or November, and then the entire sequence repeats itself. We’re currently in that longest possible transitless gap.
Also on Mars, all shadows are much crisper than they are here. They have harder edges thanks to the smaller-appearing Sun. This seldom-mentioned visual oddity would make the martian surface feel deeply alien. (Its salmon-colored sky boosts the weirdness factor, too.) By contrast, shadows on Mercury are twice as blurry as those on Earth. Those shadows are strange in the opposite way.
Do a groundhog impersonation and inspect your own shadow. Your ankles’ shadows have sharp edges. Newton used the “sharp-edged” business to argue that light is a stream of particles — if light were waves, he said, it would cast indistinct shadows. (Newton was both right and wrong because light can be a wave
and a particle.)
But your head
does cast a fuzzy-edged shadow. This isn’t a lingering effect from last night’s debauchery. It’s due to the longer distance to the ground from your head versus your ankles. After a few feet of shadow-travel, the Sun’s 1/2° apparent size becomes increasingly important. Your head’s blurry shadow edge is the zone where the Sun is
partially eclipsed.
Punxsutawney Phil loves this kind of stuff. We might as well get on board.
Contact me about my strange universe by visiting http://skymanbob.com.