What would it be like to observe the Earth from the Moon? We suit up for a look!
One of my favorite photos shows astronaut Harrison Schmitt standing next to the American flag with the Earth in the background during the Apollo 17 mission. Taken by Eugene Cernan, it's one of the few images that features our planet and a fellow human together on another world.
I ran across the photo again recently and wondered if I could recreate the scene and get a feel for what it would be like to stand on the Moon and see our home planet from afar using planetarium-style software. How bright is the Earth? How does it move in the lunar sky?
Because the Moon is tidally locked to the Earth — it revolves around our planet at the same rate that it rotates — we always see the same side. While the Moon's gravity has acted as a brake on Earth's rotation over time, our Earth still spins merrily away in the lunar sky every 24 hours. Observers would see ever-changing cloud patterns, the blue oceans, polar ice, and the general outlines of the continents coarsely with the naked eye and much better in a pair of binoculars.
If you've observed Mars through your telescope and watched as the planet's rotation brings new features into view, observing the Earth under magnification would feel oddly familiar since both planets have similar rotation rates, with the Martian day or "sol" just 40 minutes longer.
Standing on the lunar equator, the Earth would shine near the zenith and cycle through phases the same way the Moon does on Earth, at the same time describing a small loop in the sky (more on this in a minute). Earth's phases are complementary to those of the Moon: when the Moon is new (from an earthling's perspective), would-be Lunarians see a full Earth. Because the near side of the Moon always faces the Earth, I used to think Earth stayed put in the lunar sky, its altitude varying depending on location: overhead at the equator, midway up from the mid-latitudes, and scraping the horizon at the poles. But yeah, the Earth moves!
The Moon's orbit is inclined 5.1° to the plane of our planet's orbit which causes the Earth to weave north and south of the ecliptic during each lap around our planet exactly as the Moon does in Earth's skies. The Moon also moves faster in its orbit when closer to the Earth and slower when farther away, making it appear to rock from side to side each month and exposing areas beyond the east and west limbs.
The Moon's tilt combined with the its changing speed along its orbit causes the Earth to trace out a small ellipse in the lunar sky approximately 15° long each lunar revolution. Earth travels along this ellipse at a varying rate, reflecting the Moon's speed-ups and slow-downs along its eccentric orbit. Seen from the polar regions, where the Earth is always near the horizon, the planet would routinely rise and set within a small patch of sky each orbital cycle. Weird.
Just as the Moon's brightness varies with phase, so does the Earth's — a day-old terrestrial crescent shines around magnitude –6, while the Full Earth glares at –16, fifteen times brighter than the full Moon. When the Moon is new and draped in darkness, the full Earth provides a useful source of nighttime illumination, enough to pick your way among the lunar rocks . . . provided you were dressed for the –200°C temperatures.
The Earth reflects considerably more blue light thanks to the oceans compared to what the Moon sends our way — I suspect an astronaut would notice a watery hue to the ubiquitous grey rocks and distant hills. This would modulate to a dusky red-orange during the infrequent times when the Earth hid the Sun during a total solar eclipse — the atmosphere would glow with the ruddy radiance of a million sunrises and sunsets and drape the moonscape in subtle, warm hues.
Given the Earth's greater size, it naturally appears larger in the lunar sky. From the perspective of an astronaut standing on the Moon's surface, Earth varies from 1.8° to 2° in apparent diameter as the Moon travels from perigee (closest approach) to apogee (farthest) during its 27.3-day orbit.
From Earth, the Moon appears to move about one outstretched fist to the east each night, hopscotching from one zodiac constellation to the next, but from the Moon the Earth remains in one small part of the sky as the constellations march by. And those amazing planetary conjunctions we so look forward to? Exactly as the Moon passes one planet after another in conjunction each month so too does the Earth. With a difference. Like subjects approaching a medieval king, the planets come to the waiting Earth, hold audience, and then depart.
The orientation of Earth's axis with respect to the Sun changes throughout the year. Around June 21, the north polar axis tilts maximally sunward, while around December 21 the south polar axis has its turn. Best times for viewing the Earth's polar regions from the Moon occur when the Moon is new (Earth is full) during Southern and Northern Hemisphere summers. Both times, the ends of the Earth are tipped in the Moon's direction and in sunlight.
Let's return to the Apollo 17 photo again. Astronauts Gene Cernan and Harrison Schmitt landed in the Taurus-Littrow highlands on the afternoon of December 11, 1972, Eastern Time. According to NASA's Apollo 17 mission timeline, the flag was deployed and documented a little more than an hour into the astronauts' first EVA (Extravehicular Activity), or at 1:13:58 GMT December 12, 1972, to be precise. The Apollo Lunar Surface Journal gives the Sun's altitude during the "flag raising" as approximately 16°.
These crucial bits of information allowed me to verify the simulated view of the lunar sky created with the sky-charting program Stellarium. On that December day, Earth was in waning gibbous phase 3° southwest of Regulus in Leo some 45° high in the southwestern sky. At magnitude –15, the glare probably made the star difficult to see unless you raised your hand and covered our planet. The Sun shone low in the eastern sky in Ophiuchus far off to the left in the simulation. Given the Moon's location in the photo and the glint of sunlight on Harrison's visor, my hunch is he was facing north-northeast when Cernan snapped the image.
If you are a Moon lover like I am, Saturday October 20th is International Observe the Moon Night, a worldwide celebration of lunar science and exploration. Italian astronomer Gianluca Masi will offer a live, online observing session where viewers can watch the Moon rise above the Roman skyline. How cool is that? Click here for details.
If you have good weather, take a walk some night this week, look up at the Moon and use it to get a better view of the planet you're standing on.