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Moon, Mercury, Venus July 14-16

The young moon swings close to the planets Mercury and Venus over the next few days. However, it’ll be quite a challenge to spot the slender crescent pairing up with Mercury, the solar system’s innermost planet, after sundown on July 14. Because Mercury is waning in our evening sky, it is also dimming day by day. So you may need binoculars to spot Mercury in the glow of evening dusk, especially from mid-northern latitudes (US, Canada, Europe and northern Asia).

Given clear skies and an unobstructed horizon in the direction of sunset, it’ll be much easier to view Mercury with the eye alone in the Southern Hemisphere. Mercury stays out later after dark at southerly latitudes. For instance, at mid-northern latitudes, Mercury sets about one and one-quarter hours after sundown; at the Earth’s equator (0 degrees latitude), Mercury sets about one and three-quarter hours after the sun; and at temperate latitudes in the Southern Hemisphere, Mercury sets more than two hours after sunset.

Click here for a sky almanac giving you Mercury’s setting time in your sky

Watch for the crescent moon to pair up with Venus on July 15, and then to move above Venus on July 16. With the unaided eye or binoculars, see if you can spot the soft glow of earthshine – twice-reflected sunlight – on the nighttime side of the moon.

Although the phases of Mercury are only visible through the telescope, Mercury’s phase impacts this planet’s overall magnitude (brightness). Mercury’s maximum brightness happens near full phase and minimum brightness occurs near new phase. But Mercury’s visibility also depends on this world angular separation from the sun, which is presently 26o east of the setting sun.

Want to know Mercury’s present elongation from the sun? Click here and look under the last column on the right, entitled solar elongation. You can also seek out Mercury’s apparent magnitude in the middle column.

Day by day, Mercury is dimming in the evening sky because this world is waning toward new phase. Mercury will reach new phase on August 9, 2018, to transition out of the evening sky and into the morning sky.

Unlike Mercury, Venus’ greatest brightness doesn’t occur at full phase but when its disk exhibits a crescent phase (about 25% illuminated by sunshine). Venus is now displaying a waning gibbous phase. Although Venus is now waning in the evening sky, the sky’s brightest planet is also brightening. Venus will reach its greatest brilliancy as the evening “star” in September 2018.

It seems odd that one of these inferior planets – planets that orbit the sun inside of Earth’s orbit – is brightest at full phase (100% illuminated) while the other appears most brilliant in our sky at about 25% illumination. Apparently, this difference is due to the fact that Venus is covered over by clouds whereas Mercury is a rocky world much like our moon.

Find an unobstructed horizon in the direction of sunset. In the deepening dusk, watch for the young moon to swing by Mercury on July 14 and then Venus on July 15.

Bruce McClure

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