Mercury -speedy - but easy to see if . . .
The next couple of weeks are our best opportunity to see the fleeting planet, Mercury, low in the western sky right after Sunset. This last week in May might be the best time to see it. While I hope folks get an opportunity to see it in a telescope, it's just fun to catch with your naked eye or binoculars.
Here's a map for tonight - May 26, 2007 - but is roughly good right up to about June 10. Click to enlarge.
I don't know when I first saw Mercury – I suspect I had been an amateur astronomer at least 10 years – and possibly 20 – before I did. That's in part because I believed all the talk about how difficult it is to see. It's not difficult. You just have to be looking in the right place at the right time and know what to expect.
The right place now is low in the north west, about 10 degrees to the left (one fist) of where the sun just set. Another way to find this place is to draw an imaginary line from Venus – very bright even during twilight – to where the sunset. Mercury will be about two fists length down this line from Venus, toward the Sun.
Time is absolutely critical. The sun sets shortly after 8. Wait a half hour after sunset,then start looking. You need a clear western horizon, of course, because Mercury is between one and two fist lengths above the horizon. The problem here is right after sunset Mercury is at its highest – BUT the sky in that area is still very bright. As the sky darkens it becomes easier to see Mercury, BUT the planet is getting closer to the horizon, so you are looking through more obscuring atmosphere making it dimmer.
What you are looking for is the first "star" to emerge in that region. Well, of course, the first will be Venus, but it's much higher and much brighter. Mercury is just a tad brighter than Saturn. In fact, Saturn is above Venus by almost three fists along the same imaginary line that connects Venus, Mercury and the Sun. This line, by the way, is roughly equivalent to the ecliptic – the annual path of the Sun in the sky, and all the planets will always be somewhere near it.
I scan in the general vicinity where I expect to see Mercury with binoculars and invariably pick it up in them first. About five minutes later I can see it with the naked eye. By 9:05 – roughly an hour after sunset – Mercury will be difficult to see because it will be less than 10 degrees above the horizon. In another half hour it will be gone.
Conditions for seeing Mercury will improve until they peak about June 2. It will still be reasonably easy to spot up until about June 10.
This is typical of the behavior of Mercury. We get a fleeting opportunity in either the evening or dawn sky, then it vanishes – and all opportunities are not created equal. So is it hard to see? No. But you do have to be looking in the right place at the right time for the right thing.
Good luck!