These evenings in May, 2015 are a good time to look for the planet Mercury. It reaches its greatest elongation – greatest angular distance, east of the sun – on May 6 (or May 7, depending on time zone). That means this world can now be spotted in the sunset direction as dusk ebbs into darkness. Cloudy tonight? No worries. Look for Mercury any evening in the first few weeks of May.
At elongation, Mercury is 21 degrees east of the sun. The time of elongation is May 7 at about 5 UTC, but that time doesn’t matter. We all see Mercury in the west after sunset.
Mercury is the solar system’s innermost planet and orbits the sun inside of Earth’s orbit. That’s why Mercury always stays close to the sun in Earth’s sky. It’s often lost in the sun’s glare, but not now.
Here’s how to find Mercury. For people all over the world, the planet Venus is the first “star” to pop out after sunset. As the sky darkens, you’ll see bright Jupiter appear above Venus. Venus and Jupiter rank as the second- and third-brightest celestial objects to light up the May evening sky, after the moon. Simply draw an imaginary line from Jupiter and past Venus to locate Mercury toward the sunset point on the horizon (see sky chart, above).
Start looking for Mercury on any clear evening around now shortly after sunset. With binoculars, you might be able to nab this planet 30 to 45 minutes after sundown. Or see it with the unaided eye by around 45 to 60 minutes after sundown. Given an clear sky and unobstructed horizon, you might see Mercury as long as 30 minutes with the eye alone before it sinks out of sight. If you can, find an an unobstructed in the direction of sunset to maximize your chances of viewing Mercury, the innermost planet of the solar system.
At mid-northern latitudes, astronomical twilight ends – and it gets really dark out – nearly two hours after sunset. At about that same time, Mercury sets beneath the horizon.
We should mention that the Northern Hemisphere enjoys the better view of this particular evening apparition of Mercury. That’s because the ecliptic – the pathway of the planets – hits the horizon at a steeper angle as the sun sets in the Northern Hemisphere sky. The result is that Mercury stands higher over the horizon at sunset in Northern Hemisphere than at comparable latitudes in the Southern Hemisphere.
For instance, at 40o north latitude – the latitude of Philadelphia, Pennsylvania – Mercury’s altitude at sunset is about 20o. In contrast, at 40o south latitude – the latitude of Wanganui, New Zealand – Mercury’s altitude is about 9o at sunset.
No wonder Mercury sets more than 100 minutes after sunset at mid-northern latitudes, but roughly 60 minutes after sunset at mid-southern latitudes.
The farther north you live, the later that Mercury sets after sunset; and the farther south you live, the sooner.
Still, if you’re in the Southern Hemisphere, look for Mercury. Use your binoculars! You can see it, too. The photo below shows Mercury over in Rio de Janeiro, Brazil, on the evening of May 3, 2015. It’s from our friend Helio C. Vital.
Bottom line: Mercury reaches greatest elongation – greatest angular distance east of the sun – on May 6, 2015. That makes this a good time to look for this world after sunset. It’s more easily visible from the Northern Hemisphere than the Southern, but people across the world have a shot at seeing it. Mercury will linger in our evening sky until mid-month for the Southern Hemisphere and until the third week in May for the Northern Hemisphere.