The planet Mercury is at its greatest eastern (evening) elongation today, on October 9, 2013. At greatest elongation, Mercury reaches its greatest angular distance east of the setting sun. That means Mercury is now an evening “star” in the western sky after sunset. However, this particular apparition greatly favors the Southern Hemisphere, as the two charts on this page show.
The chart at the top of the post shows the Northern Hemisphere view. See how the ecliptic – pathway of the planets – is making a narrow angle with respect to the horizon? That’s always the case on autumn evenings, no matter which hemisphere you’re in. When the ecliptic makes this narrow angle with respect to the horizon, Mercury’s distance from the sunset is mostly sideways along the horizon. So Mercury stays low in the sky and sets soon after the sun. Notice that the chart at the top of this post shows a twilight sky.
In contrast, right now in the Southern Hemisphere, it’s springtime. And on spring evenings, from either hemisphere, the ecliptic stands nearly straight up with respect to the horizon. So Mercury’s distance from the sun places it high in the sky, clearly visible to all with cloud-free skies. Notice that the chart below, showing a Southern Hemisphere view of Mercury, indicates that Mercury stays out after dark for that part of the world.
Because it’s now spring in the Southern Hemisphere and autumn in the Northern Hemisphere, the southern climes have the much better of view of Mercury – and also of the other evening planets: Saturn and Venus.
Mercury is the innermost planet. It’s often lost in the sun’s glare as seen from Earth. But at opportune times, Mercury can be seen after sunset when this world swings farthest east of the sun, as seen from Earth. Today, it’s 25o east of the sun.
Bottom line: Mercury, the innermost planet of our solar system, is at greatest eastern elongation today. That means it’s at its great apparent distance east of the sun – visible in the west after sunset. Mercury follows the sun beneath the horizon before darkness falls as seen from Northern Hemisphere locations, but it stays out until well after dark as seen from the Southern Hemisphere. Enjoy the attraction, especially if you reside at the northern tropics or the Southern Hemisphere, as Mercury reaches its greatest evening elongation today!