One of the more pleasing aspects of astronomy is the extent to which it is utterly predictable. The North Star will never appear in the southern sky, the Southern Cross will never be visible from Virginia, and if Orion is high in the southern sky at midnight, it must be December. And yet it is the astronomical objects or phenomena that do not follow long-understood patterns which have provided the insights needed to advance our knowledge of the universe beyond our planet. One of the oldest of these phenomena is the movements of the planets in our sky. They certainly follow a pattern—or perhaps more accurately, each follows its own pattern—but ones that do not exactly correspond with that of the stars.
Six of the eight classical planets (sorry, Pluto-lovers) were known to the ancients, as they are all naked-eye objects. There is our own Earth, of course, then Mercury, Venus, Mars, Jupiter and Saturn. Mercury and Venus never venture too far from the Sun in our sky, while Mars, Jupiter and Saturn complete full circles around the sky in periods ranging from two to thirty years. The discoveries of Uranus and Neptune had to wait for the era of telescopic observations.
This means that when I am asked what time Saturn is visible in the evening sky, there is no automatic answer as there is with the Orion Nebula. I can tell you when Saturn is visible this year (April through August), or tonight (any time after dark, starting out halfway between the horizon and the zenith, to the south), but there isn’t anything like the always-correct answer for the Orion Nebula of “pretty much any time from November through February, every year”.
One can make sense of this by assuming that all astronomical objects revolve around a central and stationary Earth. It certainly looks that way. But the more closely you observe how the planets move, the more ad hoc assumptions you would be forced to make. Why do Mercury and Venus never appear on the opposite side of the sky from the Sun? Why does Mars, when it is at its largest and brightest, appear to slow down and reverse its night-to-night motion among the background stars? There is a central principle in science known as Occam’s Razor. When there are several alternative explanations for a phenomenon, the simplest explanation is most likely the best. Copernicus’ choice to put the Sun rather than the Earth at the center of his universe—that was an aesthetic choice as much as any other. Beauty in science lies in simplicity.
Right now we are the beneficiaries of one of those periodic planetary alignments that make it easy to see several planets at once. Four of the five naked-eye planets are grouped closely together in the pre-dawn skies for the next couple of weeks, and three of them for the next month or so. Sunrise is around 6 a.m. EDT in Lynchburg. The first of these four planets to rise above the horizon is Jupiter, around 4:30. (These times are for Lynchburg, Virginia and for a flat horizon; your times may differ.) Starting around 5 a.m., Mars, Venus and Mercury rise, in that order, all grouped very closely together. Venus is by far the brightest of these four, followed by Jupiter, Mercury and Mars.
Mars and Mercury will be hardest to see, both because they are fairly dim in an increasingly bright morning sky, but Mercury because it will be so close to the Sun, tending to get lost in its glare. Binoculars will help you spot it. Once the Sun has come into your view, however, QUIT USING THE BINOCULARS! I do not want to be responsible for anyone damaging his or her eyes by accidentally viewing the Sun through an unfiltered optical device.
Here is the eastern sky as seen from Lynchburg at 6 a.m. EDT:
(As always, clicking on the image will bring up a larger version of it.) The Sun is just getting ready to rise, and the waning crescent moon is above and to the right of sunrise. The thin blue line running diagonally is the ecliptic, the line along which the Sun, moon and planets will be found in our sky. Above and to the right of the moon is Neptune; below and slightly to the left is Uranus. But these are telescopic objects—you won’t see them with your naked eye.
Lower down are four planets. From right to left (and top to bottom) they are Jupiter, Mars, Venus and Mercury. These relative positions won’t change much over the next few days. Mercury will change the most. It is swinging ever closer to the Sun and will soon be lost completely to sight until it emerges on the “other side” of the Sun in the early evening sky.
You may have noticed that this covers all of the planets except Saturn! Saturn is literally on the other side of the sky, visible in early evening. To see why these planets all look close together from our point of view, and Saturn does not, take a look at this diagram:
Generated by the same software as for the previous diagram (The Sky), this looks down on the solar system from above the Earth’s north pole. The outermost planet visible at the top is Saturn. The next planet moving in toward the Sun is Jupiter, at the bottom. Earth is a green dot at about the 10 o’clock position around the Sun. The innermost planet is Mercury, followed by Venus, then the Earth, then Mars, Jupiter and Saturn.
Can you see how the four planets line up as seen from Earth? And can you see how Saturn appears in the opposite direction? The Earth spins on its axis in a counter-clockwise direction as seen from this vantage point, so these four planets come into view just before the Sun does.
Visualizing in three dimensions, and seeing the same thing from multiple perspectives, is one of those skills that not everyone possesses. I hope these diagrams help! And even if this doesn’t click for you, you can still enjoy the same planets our distant ancestors saw. Knowing how things work doesn’t make the sight any less wondrous.