Mercury puts in its best evening appearance of the year this month. And it does it in great company, right next to the planet Venus. The two will be just 1° apart this evening and remain within 5° of each other through March 22. In the coming days, Mercury will slide up to the right and above Venus as they slowly part but never stray too far from one another. You can use Venus, the brighter, easier planet, to guide your gaze to Mercury. Tonight, they’re in conjunction and closest, but they’ll remain within a couples of degrees of each other through Wednesday the 7th. This helps because we all know how weather inevitably has the upper hand when it comes to astronomical events.
To find Venus, find a place with a great view to the west. The closer to the horizon you can see the better. Venus is still low at dusk but climbing higher and becoming easier to see with every night. Provided the sky is free of hazy cirrus clouds, Venus is fairly easy to see with the naked eye about 30 minutes after sunset about 5° (three fingers held together at arm’s length) above the western horizon.
If you use binoculars, taking care to focus them first at a distance object like a cloud, you can sweep up the planet only 15-20 minutes after sunset. I did this the night I took the photo. Once visible in binoculars, I knew exactly where to look to find the planet with the naked eye. 15 minutes later I picked it up with no effort, a bright “star” simmering in the orange twilight. Mercury was very easy to spot in binoculars but took a little more effort to see without optical aid.
Both will be higher up and easier tonight and ripe for both visual observation and photography. I suggest placing your camera on a tripod, setting the ISO to 400 and shooting the scene 30-45 minutes after sunset, when the planets will stand out best. You don’t need a long exposure: try ⅛, ¼ and ½ second exposures at f/4.5. Sometimes you can get a little camera movement from pressing the shutter button which can cause the planet images to smear slightly. To avoid this and get pinpoint images, set your camera to self-timer, so there’s a delay between pressing the button and taking the picture. Works like a charm.
Both Mercury and Venus are hotter than the Earth because they’re considerably closer to the sun. Venus bakes at 872° (467°C) while Mercury’s no slouch at 800° (430°C). Mercury’s small, only about 800 miles larger than the moon, while Venus is only slightly smaller than the Earth. You’d think Venus would be the more clement planet because it’s nearly twice as far from the sun as Mercury. But its incredibly thick carbon dioxide (CO2) atmosphere traps heat incredibly well and distributes it across both the night and dayside of the planet, making it hot everywhere you go on Venus. The only slightly cooler places are found in higher elevations such as the peaks of the Maxwell Mountains which rise to 36,000 feet (11,000 meters). Mercury in contrast has no atmosphere. At night, temperatures there drop to –280° (–170°C).
In the early history of the planet, Venus had oceans like the Earth, but they evaporated due to the extra heat the planet received from the sun. Water vapor is a potent greenhouse gas, trapping energy and warming the oceans even further. With more heating came more evaporation until all the water disappeared. Meanwhile, water vapor molecules, made of hydrogen and oxygen, were busted apart by ultraviolet light from the sun. Being very lightweight and fast-moving, the hydrogen drifted off into space, while the oxygen combined with rocks on the surface, leaving CO2 as the main constituent of the air. Volcanic eruptions, as common on Venus as on Earth, added even more CO2, another heat-trapping gas, to the atmosphere until Venus became the horrifically hot place it is today.
Clear skies are a commodity never found on Venus, but when they happen here on Earth, use them to see Mercury and Venus in the west at dusk.