Sky Talk June 2007: Greatest Elongations of Mercury & Venus

Greatest elongations of the two inner planets Mercury and Venus are the points in their orbits where they appear at their maximum angular separations from the Sun — either to the east of it when in the evening sky or to the west of it in the morning sky. June offers a superb opportunity to see both worlds at their greatest eastern elongations within less than a week of each other!

On the evening of June 2nd, elusive Mercury will reach its greatest elongation some 23 degrees east of the Sun. Optimum viewing time will be about 30 minutes after sunset itself, with the planet appearing above the north-northwestern horizon.

Although easily spotted with the unaided eye for about a week around that date, Mercury quickly descends in the sky and finally disappears into the twilight glow by the 18th of the month. Binoculars are ideal for following this rapidly-moving “Winged Messenger of the Gods” throughout its apparition.

Just six nights later, on June 8th, the dazzling planet Venus reaches its greatest elongation some 45 degrees east of the Sun. This radiant “Evening Star” continues to dominate the evening sky, as it has for the past several months. Shining at magnitude -4.3 as June opens, it will increase in brightness into early July — at which time it reaches its greatest brilliancy, glowing at an amazing -4.7 magnitude. This makes it far and away the brightest of all celestial objects, being outshone by only the Sun and Moon.

In fact, throughout both months, Venus is radiant enough to actually cast shadows! This is something you can prove to yourself by facing away from the planet and placing a white surface such as a sheet on the ground in front of you. (Note that surrounding light sources like house and porch lights or street lamps can largely mask this effect.)

Both Mercury and Venus go through changing phases like those of the Moon, and these can be seen at magnifications of around 30x or more in small telescopes — ones such as the Edmund Scientifics 60mm refractor.

At the times of their greatest elongations, both planets will appear to be about half-full. Mercury then rapidly becomes crescent-shaped as it dives towards the horizon over the following two weeks. Venus does the same but takes much longer, remaining visible until the end of July. Interestingly, this planet is brightest when in the crescent phase, rather than when full as you might expect. That’s because it’s nearly on the opposite side of the Sun from us when full and, therefore, at its greatest distance. Conversely, when in the crescent phase, it’s at its closest point in its orbit to us. As a result, this huge crescent (which can actually be glimpsed in steadily-held or image-stabilized binoculars) is many times larger than its fully-illuminated tiny disk is, and so it reflects much more of the Sun’s light towards us.

–James Mullaney
Former assistant editor at Sky & Telescope and author of five books on stargazing.

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