Sky ArchiveTonight

Moon near Jupiter, plus a Venus milestone

On August 17, 2018, a glance up into the evening sky will show you a bright planet near the moon. That planet is Jupiter, biggest world in our solar system by far (more massive than all the other planets combined). The moon is now in the process of passing all four planets in the evening sky, starting with Venus earlier this week.

And speaking of Venus, you can’t fail to notice it, too, assuming you’re looking not long after sunset, and assuming your western twilight sky is clear. Venus is even brighter than Jupiter, not because it’s so big (it’s a near-twin to Earth in size and mass), but because its surface is covered with highly reflective clouds. On August 17, 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. That means Venus is now as far east of the sun as it will get for this evening apparition (when you’ll find Venus in the western sky).

As seen from Earth, Venus resides a maximum of 46 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. On 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 46 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 Venus sets after sunset; and the farther south you live, the later Venus sets after the sun. The reason for the discrepancy has to do with the tilt of the ecliptic – Earth’s orbital plane projected onto the great dome of the 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 orbit 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. It’s late winter/early spring now in the Southern Hemisphere … voila, Venus is high in the sky after sunset as seen from there. Southern Hemisphere sky watchers will see Venus staying out until after dark until around mid-October 2018.

Meanwhile, the final month of summer is coming up for the Northern Hemisphere, so Venus sits rather low in our western sky at sunset. At present, from northerly latitudes, Venus follows the sun below the horizon as dusk gives way to nightfall. By late September 2018, Venus will probably disappear into the glare of evening twilight.

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 it does in the Northern Hemisphere.

Bottom line: The bright object near the August 17, 2018, moon is Jupiter. Meanwhile, nearby Venus swings to its greatest angular distance from the sun on our sky’s dome. This event is called Venus’ greatest elongation. Venus is now 46 degrees east of the sun.

August 17, 2018
Sky Archive

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Bruce McClure

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