As an astronomer learning the universe past Earth, I’m keen on “outdoors the field” views virtually by default. However certainly one of my favorites is every time a spacecraft takes a snapshot of our dwelling world and the moon from a terrific distance. Caught right here on the planet’s floor, our mundane perspective is never challenged, so seeing each our world and its lone pure satellite tv for pc facet by facet is a uncommon reward, a jolt to our cosmic complacence.
Generally the picture reveals each objects intimately, because the DSCOVR satellite tv for pc did in 2015. Generally the Earth and moon are fuzzy and pixelated, like they have been in a shot from 2003 by the European House Company’s Mars Specific mission, giving some sense of distance. Extra typically they’re simply dots, factors of sunshine which might be virtually misplaced among the many stars, because the Psyche mission noticed from 290 million kilometers away in July 2025. The smaller we seem in these photographs, the better it’s to understand simply how deep area is and the way we’re really afloat on an ocean of black.
One other, even rarer view is to see our planet from the floor of one other world—one thing, actually, that first occurred solely in 2004, when NASA’s Spirit rover on Mars snapped a picture of Earth in that alien sky. Within the black-and-white shot Earth is hanging within the pearly glow of the Martian twilight, the hilly horizon silhouetted on the backside. It’s a picture that just about begs us to marvel how it could really feel to face there on the Purple Planet and see it for ourselves. What wouldn’t it seem like to our eyes? Might we additionally see the moon subsequent to our blue world?
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Earth seems as a scarcely seen dot on this first-of-its-kind picture of our dwelling planet, as seen by NASA’s Spirit rover from the floor of Mars on March 8, 2004.
As traditional while you’re trying on the sky, what you see is determined by while you’re trying. Earth and Mars each orbit the solar at totally different charges, and that adjustments the attitude drastically. At their absolute closest—once they’re on the identical facet of the solar—Mars and Earth are about 55 million km aside. Once they’re on reverse sides, although, that distance stretches to 400 million km!
This influences two essential elements in viewing Earth and the moon as a pair within the Martian sky: how far aside they seem, and the way brilliant they’re.
You’d assume Earth and the moon can be brightest once they’re closest to Mars as a result of brightness is determined by distance. However this isn’t the case! When the 2 planets are closest collectively, they’re on the identical facet of the solar, placing Earth between Mars and the solar. Which means that, from Mars, you’d be Earth’s unilluminated night time facet. That’s darkish, clearly, making Earth seem dim.
When Earth is reverse Mars from the solar, it’ll seem full since you’re trying instantly on the daylit facet. However mitigating that is its far better distance, plus the truth that Earth seems a lot smaller within the sky. So, irritatingly, when it’s closest to Mars, it’s faint, and when it’s farthest from Mars, it’s additionally faint!
There is a contented medium, nonetheless. As Earth orbits the solar, from Mars, it’ll seem to undergo phases, very like the moon does as seen from Earth. This phasing is determined by the viewing geometry: Earth is full on the other facet of the solar and “new” (just like the darkish new moon) when it’s between Mars and the solar. In between these two extremes our planet will seem as a crescent or half full or gibbous (between half and full) over the course of that orbit. Word that the moon will present the identical part as Earth; as a result of we’re trying from Mars, the angle between Earth and the solar is actually the identical as it’s for the moon and the solar, so their phases are equivalent.
Surprisingly, Earth will seem brightest when in its crescent part. Though it seems slimmer, this occurs when the crescent Earth is nearer to Mars, making it seem greater within the sky. The scale on the sky of the illuminated portion you see is definitely greater than when it’s full, making it proportionally brighter.
That is precisely the identical state of affairs we see from Earth after we have a look at Venus within the sky. Venus additionally undergoes phases and seems brightest to us when it’s a fats crescent. This implies the perfect time from Mars to see Earth and the moon collectively is just a few weeks after their closest method. They’ll be brighter and nonetheless well-separated within the sky.
How brilliant will they be? From Mars, presently, Earth might be at a magnitude of about –1 or –2 at greatest, which is about as brilliant as Jupiter seems in Earth’s night time sky. That’s not as flashy as Venus however nonetheless fairly good and straightforward to identify. The moon might be a “star” close by at a magnitude of roughly 2 or 3, about the identical obvious brightness as the celebrities within the Large Dipper. (The brighter a celestial object, the decrease its magnitude quantity, within the reverse-logarithmic scale utilized by astronomers.)
However brightness isn’t all there’s to contemplate right here. If you wish to see each Earth and the moon, they should be separated sufficient within the sky to be discerned individually. In the event that they’re too shut collectively, the human eye will blur them collectively right into a single object. How far aside they seem within the sky is determined by their distance from Mars, in fact—the nearer they’re to Mars, the farther aside they’ll be—nevertheless it additionally is determined by the place the moon is in its orbit as seen from Mars; because it circles round Earth, their separation will seem to alter as nicely. However let’s assume we now have good luck and occur to see them as geometrically far aside as attainable.
When the pair is closest to Mars, at most, Earth and the moon might be just below half a level aside within the sky (astronomers measure measurement on the sky in levels, the place there are 90 levels from the horizon to the zenith). That’s simply sufficient to see them as two distinct objects, even with the brightness distinction. Once they’re on the other facet of the solar, they’re solely an eighth that distance aside, making them way more tough to differentiate. If we select the time once they seem brightest, as thick crescents, they’ll be separated by about one third of a level—far sufficient aside to see individually. There’s nonetheless another difficulty, nonetheless.
Earth is about 4 instances wider than the moon and much shinier; our floor and cloudy air replicate daylight about 3 times higher than the moon’s grey floor. Altogether, this implies Earth is roughly 50 instances brighter than the moon when seen from a distance. This makes distinction an issue; they should be far sufficient aside within the sky that the moon isn’t misplaced in Earth’s glare!
Placing all this collectively, if the orbital geometries all align simply so, I discover that the moon and Earth will certainly be seen within the Martian sky as two separate objects, although you might need to squint a bit to see the a lot fainter moon. When you have binoculars, you would possibly be capable of spot each the moon and Earth sporting the identical part. A telescope would reveal Earth’s continents and oceans; actually, Earth’s brightness and even its colour would possibly change because the planet rotates or while you see a cloudy versus a transparent day.
In spite of everything this, I’ve to marvel: When will certainly one of us really stand on the dusty floor of Mars and see Earth hanging within the gloaming close to the horizon? And even additional off: In the event that they’ve been on Mars lengthy sufficient, will they have a look at that gleaming beacon within the sky and nonetheless see it as dwelling?
Many due to my good friend (and lecture agent) Beth Quittman for asking me this query.
