The first days of February offer your best chance to see all of the naked-eye planets — from Mercury to Saturn — together with the Moon in the predawn sky.
A few weeks ago, some websites started touting how you could spot all five naked-eye planets in the predawn sky at once from January 20th to February 20th. I even saw a short announcement about it on a national news broadcast back then.
But no one saw all five on January 20th, because Mercury was unobservable — even though, technically, the innermost planet was above the horizon before sunrise on that date. In fact, spotting Mercury low in the predawn twilight by eye was a challenge on January 25th, five days later.
Here's why: First, right now Mercury is between us and the Sun, and it's swinging toward what's termed greatest elongation (26° west of the Sun) on February 6th. Back on January 20th the planet's apparent separation from the Sun was only half that, about 13°. So once Mercury popped into view above the southeastern horizon, twilight was already very strong and sunrise just minutes away.
Second, a close-up of Mercury would show that the planet's tiny disk is waxing (becoming more fully illuminated) day by day. So its apparent brightness is increasing. And, third, due to its low altitude, we're seeing Mercury through lots of extra atmosphere and that makes it dimmer by roughly 25%.
Look at the image below, shot by my S&T colleague Sean Walker 45 minutes before local sunrise on January 28th. Mercury (at center left) looks awfully dim — at that moment it was 6½° above the horizon and 10° from Venus (upper right):
And yet, this morning I had no trouble spotting Mercury by eye while heading in to the office. In fact, all this week this little planet should be its easiest to spot.
First, however, make sure you have a really clear sky and an unobstructed view toward southeast. Second, don't wait too long. Be looking an hour to 45 minutes before sunrise — that's the "sweet spot" for Mercury's visibility. Here in Boston, right now this window corresponds runs from 6:15 to 6:30 a.m.; find your local time of sunrise here.
Once you've glimpsed Mercury (and Venus), the other planets are a snap. Well to the upper right of Venus, by about three times the width of your fist at arm's length, is creamy-colored Saturn. (The slightly dimmer star to Saturn's lower right is Antares, in Scorpius.). Shift your gaze farther right to sweep up Mars, then the icy-white star Spica, and finally Jupiter. The arc from Mercury toward Jupiter is roughly 115° in all.
And the Moon has joined this planet party! It was near Jupiter on the mornings of January 27th and 28th, and then Mars on February 1st. Look for it near Saturn on February 3rd, Venus on February 5th, and above Mercury on February 6th. (In fact, the clustered crescent Moon, Venus, and Mercury should be very pretty on those final two dates.)
Seeing all five planets at once isn't particularly rare — it last occurred 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.