Perhaps you caught the moon last night in the southwestern sky – a thin crescent lit by sunlight. If your timing was right, with the sky not too bright and moon not too low, you may have also seen the entire outline of the moon. This dusky portion is illuminated not by direct sunlight but by sunlight reflected off our planet into space in the moon’s direction. It strikes the moon and bounces back to our eyes. Because Earth is the source of the light, we call it earthlight.Â After getting reflected twice – first off Earth and then the moon – it’s considerably fainter than sunlight, yet bright enough that we can discern a number of craters and lunar seas through binoculars and small telescopes. The earthlit portion of the moon is most easily visible during crescent phase. If you missed it last night, it’ll be out again tonight.
Last week we talked about the light coming from the moon being “fresher” than that from Venus. The moon is much closer to us than Venus; sunlight reflected from its globe takes only 1.3 seconds to reach our eyes compared to several minutes for Venus.
I got to thinking about the time it takes light to travel distances and realized a most curious thing would occur should the sun ever go dark. Of course this isn’t likely to happen. Astronomers forecast another few billion years of bountiful sunshine before internal changes in our star’s core cause it to bloat into a red giant, cast off its outer layers and settle into an even longer life as a white dwarf. Let’s pretend for a minute that you could turn it off with a switch. Further, we’ll imagine it’s evening and we’re outside watching the crescent moon as the stars and planets begin to come out.
Click! Just like that, the sun suddenly goes dark. We’d still have to wait 8 minutes to find out it happened, because that’s how long it takes light to travel the sun-Earth distance (93 million miles) at 186,000 miles per second. When the darkness hit, twilight would suddenly be over as Earth went dark. But wait. The moon is still visible – what the? Sunlight leaving the illuminated crescent the instant before all went dark is still streaming to Earth. Despite our rising terror, we’d relish 1.3 seconds of additional moonlight – and earthlight – before the crescent also went dark.
Here’s the weird part. Because reflected light leaving Earth for the moon takes 1.3 seconds to get to the moon and another 1.3 seconds to make the return trip – a total travel time of 2.6 seconds – the earthlit portion of the moon hangs there all by itself for an additional 1.3 seconds before it finally succumbs to darkness. Once the entire moon went dark, its presence would be betrayed only if it happened to pass in front of a star – causing the star’s light to blink out – otherwise you’d have an extremely difficult time finding the small, “empty” circle in the sky taken up by the dark moon.
What about the planets? The nearer ones like Mercury and Venus would still gleam for a few minutes before they, too disappeared. Mars would stick around for about a half hour as the last beams of sunlight reached the planet and then were reflected back across the gulf separating our two planets. Jupiter would remain in view about an hour, Saturn a couple hours and the most distant planet Neptune about 8 hours before the outgoing “wave of darkness” snuffed out each planetary tiki lamp in turn. Of course I’d get the scope out and try to see every one of them one last time before they disappeared from view.
Mind you, even if we could turn the sun off, the planets would still remain in orbit, the moon would circle the Earth and so on. If you removed the sun entirely, then without gravity’s tug, all the planets would go flying off in straight lines like swinging buckets let go.
It’s fun to play out odd scenarios because of what they can teach us about our solar system and the properties of light. If the sun were to really shut down what would happen anyway? According to an article in the online Popsci, the average global surface temperature would drop below 0 degrees Fahrenheit in a week and to -100 within a year. The top layers of the ocean would freeze and insulate the bottom from freezing for hundreds of thousands of years. Iceland would head up a list of great places to live because of its abundance of geothermal heat. Me, I’d take the family on a permanent vacation to Yellowstone National Park and pitch our tent next to one of those hot springs.Â I’ve always wanted to see the geysers.