Mercury, the closest planet to the sun, never strays far from the sun in Earth’s sky. Because Mercury circles the sun inside of Earth’s orbit, we can only see this world when it swings farthest away from the setting or rising sun. At these opportune times – about six times each year – Mercury is visible either shortly after sunset or before sunrise.
On March 14, 2014, Mercury reaches its greatest western (morning) elongation from the sun. At 27.6o west of the sun, this is the farthest that Mercury gets from the sun in all of 2014. Even so, this is a rather poor morning apparition of Mercury for the Northern Hemisphere, with Mercury rising only about one hour before the sun at 40o north latitude, the latitude of Denver, Colorado, or Toledo, Spain.
On the other hand, this is a most wonderful morning apparition of Mercury for the residents of the Southern Hemisphere. At temperate latitudes in the Southern Hemisphere, Mercury rises two hours or more before the sun.
When it occurs in the morning, a greatest elongation of an inferior planet (Mercury, Venus) is most favorable when it happens in the late summer or early autumn. That’s why the Southern Hemisphere has such a great view of Mercury in the March morning sky right now. The upcoming March equinox will be the Southern Hemisphere’s autumn equinox.
When it occurs in the morning, a greatest elongation of an inferior planet is always unfavorable in late winter or early spring. That’s why Mercury is so much harder to catch from northerly latitudes. The upcoming March equinox ushers in the Northern Hemisphere’s spring equinox.
Mercury resides in the morning sky all through March 2014. Those at southerly latitudes can easily catch Mercury before dawn. But those of us at northerly latitudes this month might need to scan with binoculars in order to spot Mercury in the glare of morning twilight.
Bottom line: Tomorrow morning – March 14, 2014 – Mercury will reach its greatest western (morning) elongation from the sun. It’s a good one for the Southern Hemisphere! It’s a not-so-good one for us in the north.