A great time for greatest elongations

¡SkyCaramba! Weekly astronomy blog for the week ending March 8, 2014

March 2014 is a great time for greatest elongations. I’m talking about Mercury and Venus being at their highest above the sun in the morning sky.

The angle between an object and the sun as we see them is called elongation. A planet that’s rising as the sun is setting has about 180° of elongation. After all, it’s about halfway around a 360° dome. If the planet’s high overhead at sunrise or sunset, it’s about a quarter of the way around and so it has about 90° of elongation. An object with 0° elongation could be between us and the sun. It could also be on the other side of the sun. The moon has 0° elongation during a total solar eclipse.

Venus and Mercury never get close to 180° elongation. Instead, they reach what’s called greatest elongation. That’s when they’re at the greatest angle from the sun before turning around and going the other way. Mercury will be at greatest elongation on March 14 and Venus will be on March 22. Both are in the morning sky.

Because Mercury is closer to the sun than Venus, its greatest elongation is less than Venus’s. And because Mercury’s orbit is more of an ellipse than Venus’s, its angular distance from the sun varies a lot from one greatest elongation to the next.

The farthest Mercury ever gets from the sun during greatest elongation is almost 28°. Its elongation on March 14 is 27.6° west of the sun. Venus gets as far as 47° from the sun. It will be 46.6° west of the sun on March 22. These two greatest elongations are truly among the greatest elongations we can see!

For Mercury, how great a greatest elongation is depends on the time of year. March and April are best for when the planet’s visible in the morning. The best evening viewing for Mercury is when it’s at greatest elongation in July, August, and September. The worst of Mercury’s greatest elongations are those that happen in September and October for morning viewing, and in February for evening viewing. At those times, the planet will be just 18° from the sun.

For Venus, every greatest elongation is pretty good. They’re always about 45° to 47°, morning or evening, any time of year. But to satisfy everyone’s curiosity, I’ll inform you the best are in January for morning events and in November and December for evening. The worst (and I emphasize that they’re not actually bad at all) are in late June and July for morning elongations and May to early June for evening. Star Venus watching and you’ll eventually agree the planet never puts on a bad show.

Well, take a look at Mercury and Venus for yourself this month and enjoy two really good greatest elongations.


The graphic for this page was made with Home Planet.

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