See Mercury, the Elusive Planet

[CBass]

Mercury just passed superior conjunction on April 16, but in the days to come it will bolt out to become easily visible low in the west-northwest at dusk. On Wednesday evening, April 23, Mercury should be visible within about 30 minutes after sunset if your sky is quite clear. Mercury will be shining at magnitude �1.6, slightly brighter than Sirius (the brightest of all stars). In fact, at that particular hour of the day, Mercury will be the brightest object in the sky!


So, if your sky is free of any horizon haze and there are no tall obstructions to your view (like trees or buildings) you should have no trouble in seeing it as a very bright "star" shining with just a trace of a yellowish-orange tinge. By April 30, Mercury will be setting as late as 85 minutes after the Sun. That evening, binoculars may show the Pleiades star cluster 4 degrees directly above it. (Your clenched fist held at arm's length measures about 10 degrees in width.)


In the evenings that follow, Mercury will slowly diminish in brightness, but it will also slowly gain altitude as it gradually moves away from the vicinity of the Sun. This is just the start of Mercury's best apparition of the year for mid-northern viewers. On the evening of May 6, be sure to look for a delicately thin sliver of a 1.5-day old crescent Moon sitting just a couple of degrees above and slightly to Mercury's right.


Pinnacle: May 14


Mercury, like Venus, appears to go through phases like the Moon. Right now, Mercury is a nearly full disk, which is why it starts off appearing so bright. Mercury reaches its greatest elongation, 22 degrees to the east of the Sun, on May 14. Shining at magnitude +0.4 (as bright as the star Procyon in Canis Minor), it will set nearly two hours after the Sun. By the time it arrives at its greatest elongation, it will appear roughly half-illuminated and the amount of its surface illuminated by the Sun will continue to decrease in the days to come. So when it begins to turn back toward the Sun's vicinity after May 14, it will fade at a rather rapid pace.


In fact, on the evening of May 22, Mercury's brightness will have dropped to magnitude +1.7; only 1/20 as bright as it was on April 23. In telescopes it will appear as a narrowing crescent phase. This, in all likelihood will be your last view of it, for the combination of its lowering altitude, plus its descent into the brighter sunset glow will finally render Mercury invisible by the final week of May.