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Disappearing Acts
StarDate:
September 2, 2019Two close neighbors are hiding in the sunlight right now. Both of them are passing behind the Sun as seen from Earth. And both will return to view next month.
Mars is going first. It’s passing behind the Sun today. And Mercury will follow it tomorrow.
A planet’s passage on the far side of the Sun is called a conjunction — an alignment of the planet, the Sun, and Earth. And in Mercury’s case, it’s called a superior conjunction.
The difference in terminology is a result of the planets’ different locations. Mars is the fourth planet from the Sun. Since Earth is the third planet, Mars always stays outside Earth’s orbit. So it has only one type of conjunction — when it passes behind the Sun.
But Mercury is the Sun’s closest planet, so its orbit is inside Earth’s. So it can have two kinds of conjunctions: when it’s behind the Sun, as it is now, and when it passes between Earth and the Sun. That alignment is known as inferior conjunction.
After its conjunction, Mars will just begin to peek into view in the dawn sky in early October. It’ll climb higher throughout the month, and will be in good view by the end of October.
Mercury, on the other hand, will climb into the evening sky around the middle of October. It won’t get all that high, though, so it’ll be tough to find. Skywatchers at southern latitudes will have the best chance to see it — during the planet’s brief interval between conjunctions.
Tomorrow: new missions of exploration.
Script by Damond Benningfield