Beauty of a halo around the moon last night! Within its circumference were the Pleiades and Hyades star clusters, Aldebaran and Jupiter – by a hair. Halos form in high cirrostratus clouds around the moon and sun from light refracted (bent) by billions of extremely tiny six-sided, pencil-shaped ice crystals into a circle of 22 degrees radius. Sometime halos last for hours; other times like last night, the clouds blew out 10 minutes after the photo was taken. One of my favorite “sub-hobbies” in astronomy is seeing what gets snared by these occasional lunar lassos.
Yesterday we talked about Earth being closest to the sun in January versus July. The difference of a couple million miles causes the sun’s size to vary a minute amount not detectable with the naked eye. That got me to wondering about how big the sun would appear from the various planets in the solar system, so I worked up an illustration to give you and idea of the sun’s size in the sky seen from Mercury all the way out to Pluto, not an official planet, but widely beloved as one just the same.
The illustration above shows the sizes to a good approximation at the planets’ average distances from the sun. The sun on Mercury, the closest planet, would be very obviously larger and the heat rather unbearable. Venus’ sun would also be noticeably bigger if you could see it through the perpetual cloud cover. It begins shrinking at Mars and becomes positively tiny at Jupiter’s distance of almost half a billion miles. Still, if you examined the sun through a safe solar filter from an airship cruising just above the giant planet’s cloud deck, you’d be able to distinguish a tiny, brilliant circle of light one-fifth the size of the full moon.
Saturn would be more challenging with the sun half the size as viewed from Jupiter. The accepted human eye limit for discerning the shape of an object in the sky is one arc minute or 1/30 the size of the sun or full moon. Only the sharp-eyed among us would be able to see the sun as a disk from Uranus and Neptune, while Pluto would elude all.
Perhaps the most amazing thing about the shrinking sun is that it remains extremely brilliant right on out to Pluto. Not radiant like the way we see it from Earth of course, but brighter than you might suppose. At the dwarf planet’s distance of 3.6 billion miles, the sun shines at magnitude -19 or eight magnitudes fainter than here at home. Standing on Pluto’s methane frosted surface, it would shine about 240 times brighter than the full moon. That’s the power of a star.