When we look up on a clear day, we see the crisp sky of Earth’s atmosphere colored by the sun’s blue light rays scattered about by air molecules.
Earth is the only planet where we would see such a skyward sight. On another planet, we would see other colors or just the dark sky of outer space.
Mercury is this extreme. With no atmosphere, if we could gaze upward from its surface, we would look directly into space. Rather than see a sky full of stars, the sun would blaze above with a brightness that varies from six to 10 times that seen from Earth, depending on Mercury’s position in its oval orbit.
Mercury’s lopsided path around the sun also makes the sun appear up to five times as large compared to an Earthly view. Even more intriguing is that Mercury’s tight solar orbit causes its year to be only three Mercurian days. This “spin-orbit resonance” brings about a very odd effect in its sky.
At the time when this innermost planet swings closest to the sun, the sun will stop its usual apparent east-to-west motion across the sky and briefly reverse its direction. For certain locations on Mercury, the sun will rise over the eastern horizon, then reverse its motion and set back in the east, then rise again from the east! At Mercury’s Caloris Planitia (“hot basin”), the sun’s back-and-forth motion causes it to loop around directly overhead, making for a truly hot day.
For a planetary observer on Mercury, Venus would be the brightest object in the sky after the sun, appearing up to 15 times brighter than seen from Earth. This happens because it is possible to see Venus in its full phase from Mercury, fully lit by the sun; from Earth, outside the orbit of Venus, we can never see Venus completely lit.
An observer on Venus would never see the night sky. Venus is surrounded by an atmosphere so thick that by the time sunlight filters to its surface, the light is a muted orange-red. Without a doubt, its substantial atmosphere is impenetrable to starlight.
Venus’ dense blanket of air produces a runaway greenhouse effect. Temperatures at the Venusian surface are nearly 900 degrees, making it the hottest world in our solar system. If we could see the sun from its scalding surface, the sun would rise in the west and set in the east because Venus rotates backward as compared to most planets.
Lunar sky
Like Mercury, the moon has no atmosphere, so its sky is also black. The sun is so bright as seen from the barren surface of the moon that sunlight washes out the stars unless the sun is blocked from view. Without an atmosphere, the lunar temperature soars to 200 degrees above zero in sunlight and dips to 100 degrees below zero in the shade.
In spite of this temperature dichotomy, Edwin “Buzz” Aldrin, who followed Neil Armstrong onto the moon’s surface during Apollo 11, remarked that there was no temperature change inside his spacesuit when moving from shadow into sunlight.
After the sun, Earth is unmistakable in the moon’s sky, appearing four times as large as the moon as seen from Earth. Because it appears larger from the moon and since it is more reflective than the moon, Earth shines more than 50 times as bright from the moon than the moon does in Earth’s sky.
Just as the moon goes through phases, so does Earth when seen from the moon. However, Earth’s phases are opposite to those of the moon. For instance, when the moon is full, a lunar observer would see Earth at its new, unlit phase and when the moon is at first quarter, Earth would be seen as in its last quarter.
Also like Mercury, the moon is spin-orbit coupled but to Earth. In this case, however, the moon spins once for every time it circles Earth. This means that only one side of the moon ever faces Earth. Earth is always visible from the “near” side of the moon and is never seen from the moon’s “far” side. In addition, as Earth cycles through its phases, it remains just about in the same place in the lunar sky.
Like Earth, Mars does have an atmosphere but it is very thin compared to Earth’s ocean of air. Then again, unlike Earth’s blue daytime sky and orange-red sunsets, the sky of Mars is colored the opposite. The day sky of the Red Planet is a bright orange-red and when the sun sets, the sky in that direction becomes bluish, tapering off to a dark rose. Pictures of the Mars daylight and sunset sky were first taken by the Mars Pathfinder Rover and can be seen on the NASA website.
Beyond Mars, the planets Jupiter, Saturn, Uranus and Neptune have thick atmospheres that only increase in density toward the center of each. These worlds do not have what we would think of as a solid surface. Nonetheless, from the cloud tops, their many moons would dot the dark sky along with their arching rings.
SpaceX update
After a few delays from late April to mid-May, SpaceX launched its Dragon space capsule to the International Space Station on May 22. Three days later, after a series of spacecraft tests, astronauts aboard the ISS used the station’s robotic arm to grasp the capsule’s grapple fixture and attached it to the station. This action made SpaceX the first commercial venture to dock with the Space Station.
On May 26, two astronauts entered the Dragon capsule and, after taking air samples, began unloading more than 1,100 pounds of provisions and experiments. By the time Dragon undocked with the station on May 30, astronauts had reloaded it with more than 1,400 pounds of cargo to be returned to NASA.
Unlike other ISS resupply vehicles that disintegrate upon re-entry, Dragon is designed to make a safe return flight so that it can someday carry astronauts to and from the Space Station.
On the morning of May 31, SpaceX completed its historic mission when the Dragon capsule successfully splashed down in the Pacific Ocean. Only four governments had previously achieved such a triumph.
Transit of Venus
The June 5 transit of Venus was spectacular. Although the sky had been overcast for most of the day, the clouds broke just a few minutes before the transit began and we were able to watch part of the ingress of Venus over the sun with our safe solar telescopes.
Just before 6:04 p.m., a small curved notch began to move over the solar disk and quickly enlarged. When about half of Venus was over the sun, thin cusp extensions of light began to extend around the part of Venus that was still against deep space. This was caused by sunlight shining around it and through its dense atmosphere.
Clouds then interfered, but we did get occasional views of Venus’ silhouetted orb against the sun. It was my most memorable celestial sight since watching a total eclipse of the sun.
Richard Monda is an astronomer living in the Capital Region.
Categories: Life and Arts
