Tonight – March 15, 2018 – the Northern Hemisphere enjoys a favorable view of Mercury in the western sky after sunset. What’s more, you can use the dazzling planet Venus to help you locate Mercury, the solar system’s innermost planet. You might see both worlds with the unaided eye around one hour after sunset. If you see Venus, but not Mercury, aim binoculars at Venus to see Mercury and Venus taking stage in the same binocular field.
Given an unobstructed western horizon, these two worlds stay out for about 75 minutes after the sun at mid-northern latitudes (though Mercury stays out a little while after Venus sets). At the equator (0o latitude), Mercury and Venus set about one hour after sundown; and at temperate latitudes in the Southern Hemisphere, they set some 45 minutes (or less) after sunset. Click here for a recommended almanac giving you the setting times for Mercury and Venus in your sky.
Because Mercury is an inferior planet – a planet that orbits the sun inside of Earth’s orbit – this world is often obscured in the light of the sun. But today – March 15, 2018 – Mercury swings out to its greatest eastern (evening) elongation. That means Mercury reaches its greatest angular distance from the setting sun and stays out for a maximum time after sundown.
This time around, Mercury’s evening apparition greatly favors the Northern Hemisphere, where Mercury is showcasing its best evening appearance of the year. Unfortunately, for the Southern Hemisphere, this is Mercury’s poorest showing in the evening sky. Although Mercury’s greatest elongation is the same worldwide (18o east of the sun), Mercury and Venus are both easier to see from the Northern Hemisphere.
Keep in mind that the ecliptic – the pathway of the sun, moon and planets – intersects the horizon at its steepest angle (nearly perpendicularly) for the year at sunset on the spring equinox (Northern Hemisphere’s March equinox). Because it’s so very close to the spring equinox in the Northern Hemisphere right now, Mercury and Venus are almost directly above the sun at sunset, so they stay out for a maximum amount of time after the sun goes down.
Meanwhile, in the Southern Hemisphere, the March equinox is their autumn equinox. At sunset on the autumn equinox, the ecliptic hits the horizon at its shallowest angle (close to horizontal) for the year. So, at southerly latitudes, Mercury and Venus appear sideways of the sun (rather than directly above it) at sunset. For that reason, Mercury and Venus set quite soon after sundown at temperate latitudes in the Southern Hemisphere.
After this evening, Mercury will sink downward, closer and closer to the setting sun; while Venus will climb upward, away from the sunset. Whereas Venus will remain a fixture of the evening sky until October 2018, Mercury will transition out of the evening sky and into the morning sky on April 1, 2018.
Venus will finally reach its greatest eastern (evening) elongation from the sun on August 17, 2018.
After sunset this evening – on March 15, 2018 – watch for Mercury to pop out in the vicinity of dazzling Venus in the deepening evening twilight.