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Venus hits milestone on August 17

Today – August 17, 2018 – Venus hits a major milestone in Earth’s sky, as this brilliant beauty of a planet swings to its greatest eastern elongation from the sun. As seen from Earth, Venus resides a maximum of 47 degrees east of the setting sun in its present apparition as the evening “star,” which started on January 9, 2018, and will end on October 27, 2018. Because Venus orbits the sun inside of Earth’s orbit, this world appears in Earth’s western evening sky whenever it’s at a greatest eastern elongation.

Not to scale. The radius of Venus’ orbit is about 0.72 of Earth’s distance from the sun (0.72 of an astronomical unit) Venus swung to the far side of the sun (at superior conjunction) on January 9, 2018, and will sweep more or less between the Earth and sun (at inferior conjunction) on October 26, 2018. Today – August 17, 2018 – Venus reaches its greatest eastern (evening) elongation from the sun. Midway between greatest eastern elongation and inferior conjunction, Venus will showcase its greatest illuminated extent as the evening “star” on September 21, 2018.

Although Venus’ greatest eastern elongation is 47 degrees as viewed from all around the world, Venus nonetheless sets earlier after sunset at more northerly latitudes, yet later after sunset at more southerly latitudes. We give you the number of hours that Venus stays out after sunset at 45 degrees north latitude, the equator (0 degrees latitude) and 45 degrees south latitude.

August 17, 2018

45 degrees north latitude: Venus sets about 1 hour and 30 minutes after sunset

Equator (0 degrees latitude): Venus sets about 2 hours and 50 minutes after sunset

45 degrees south latitude: Venus sets about 4 hours and 5 minutes after sunset

In a nutshell, the farther north you live, the earlier that Venus sets after sunset; and the farther south you live, the later that Venus sets after sunset. The reason for the discrepancy has to due with the tilt of the ecliptic – Earth’s orbital plane projected onto the great dome of sky. By the way, you’ll always see the planets of the solar system on or near the ecliptic because the planets of the solar system circle the sun on almost the same plane that our planet Earth does.

In either the Northern or Southern Hemisphere, the ecliptic makes a rather shallow angle with the evening horizon in late summer and early autumn. Conversely, in either hemisphere, the ecliptic intersects the horizon at a particularly steep angle in late winter and early spring.

The final month of summer is coming up for the Northern Hemisphere, so Venus will sit rather low in the west at sunset. From northerly latitudes, watch for Venus to follow the sun below the horizon as dusk gives way to nightfall. At mid-northern latitudes and farther north, Venus will probably disappear into the glare of evening twilight by late September.

In the Southern Hemisphere, where it’s now late winter, the ecliptic – the pathway of the sun, moon and planets – intersects the horizon at a steep angle as darkness falls. Therefore, Venus stays out longer after sunset than she does in the Northern Hemisphere.

Meanwhile, in the Southern Hemisphere, it’s the final month of winter that’s coming up. From southerly latitudes, Venus appears higher up in the sky at sunset and stays out till well after dark. In fact, the Southern Hemisphere will probably see Venus staying out till after dark until around mid-October.

No matter where you reside worldwide, however, Venus’ greatest eastern elongation is an equal 46 degrees east of the sun.

Bruce McClure

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