Alan M. MacRobert, Senior Editor
617-864-7360 x22151, amacrobert@SkyandTelescope.com
J. Kelly Beatty, Senior Editor
617-864-7360 x22168, kbeatty@SkyandTelescope.com
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During the next two weeks, for the first time in more than a decade, you can see all five naked-eye planets — from Mercury to Saturn — together in the predawn sky. This celestial treat is relatively easy to see with just your eyes; no telescope or optical aid is needed.
Technically, all five planets are in the sky before dawn from January 20th to February 20th. However, because Mercury will be most challenging to see, Sky & Telescope senior editor Alan MacRobert advises, "Look during the last few days of January and first week of February to have your best chance for success."
The optimum time to look is about 45 minutes before sunrise; at earlier times Mercury is too low to be seen easily, and afterward the twilight sky rapidly brightens. Find a location with a clear, unobstructed view toward southeast, in the general direction of sunrise, and locate brilliant Venus fairly low above the horizon. Venus is brighter than any other planet or star — you can't miss it.
To spot Mercury, look to the lower left of Venus by about the width of your clenched fist held at arm's length. Mercury will appear as a distinct star in dawn's twilight. Be sure that no trees or buildings are blocking your view. In late January, Mercury gets slightly brighter and climbs higher (closer to Venus) in the predawn sky each successive morning.
To find the other three planets, sweep your gaze in a long arc to upper right from Venus. First comes Saturn, which is much dimmer than Venus. The bright orange-tinted star Antares shines to the lower right of Saturn. Next, as you face due south, comes slightly brighter Mars. Finally, look well past the bright star Spica to spot very bright Jupiter high in the southwest. The entire arc of five planets stretches more than halfway across the sky (an arc of 110°).
Near the end of January, the waning Moon enters the scene, and its presence can help identify the planets. It is near Jupiter on the mornings of January 27th and 28th, Mars on February 1st, Saturn on February 3rd, Venus on February 5th, and above Mercury on February 6th.
These five planets appear together in the sky two or three times per decade on average, though sometimes one or more of them is too near the Sun to be seen easily. The last instance was before dawn from late December 2004 to early January 2005, when their order in the sky briefly matched their relative order outward from the Sun: Mercury, Venus, Mars, Jupiter, and Saturn. The next occurrences, in mid-August 2016 and mid-July 2020, will be challenging to see because Mercury will be very close to the horizon.
A Plane of Planets
As you sweep your gaze from Mercury toward Jupiter, notice that all these planets line along a single arc across the sky. That's no accident. All of the major planets have orbits in roughly the same plane as Earth's orbit. We see our orbit projected as a line — the ecliptic — across the sky. The Sun always lies on the ecliptic — and our Moon is never far from it either. "It's the superhighway of planetary motion among the stars," notes Sky & Telescope senior editor Kelly Beatty.
However, this predawn grouping of planets is not an "alignment" — they are not lined up outward from the Sun. Instead, they are simply positioned in their orbits such that we see them in one portion of the sky. Here are their distances from Earth as of February 1st (astronomers use the average Earth-Sun distance, called an astronomical unit, as a handy yardstick for intra-solar-system distances:
|Mercury||0.89 a.u.||82 million miles|
|Venus||1.35 a.u.||125 million miles|
|Mars||1.37 a.u.||128 million miles|
|Jupiter||4.64 a.u.||431 million miles|
|Saturn||10.51 a.u.||977 million miles|
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