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Venus at its brightest in morning sky

This week – late November and early December 2018 – the dazzling planet Venus will be shining at is brightest best in the morning sky. Venus is always the 3rd-brightest celestial object, after the sun and moon, to bedeck the heavens. Even so, Venus’ brightness is some 2 1/2 times brighter at maximum than minimum.

Because Venus is an inferior planet – a planet orbiting the sun inside of Earth’s obit – this world shows the whole range of phases, much like Earth’s moon. But you need a telescope to view Venus as a waxing crescent phase throughout the rest of 2018. Surprisingly, perhaps, Venus shines brightest when its disk is about 25% illuminated by sunshine.

Up before dawn? Then look for the bright stars Spica and Arcturus. These stars will disappear in the bright morning twilight, whereas Venus will shine on!

Keep in mind that when Venus’ disk becomes 100% illuminated in sunshine some 8 1/2 months from now (August 14, 2019), Venus will be nearly 5 times farther from Earth than it is right now. Hence, the illuminated portion of Venus’ disk will actually cover less square area of sky and Venus will be less bright at that time.

As Venus comes closer to Earth in the evening sky, its phase shrinks but its disk size enlarges. The converse is also true. Whenever Venus gets farther away from Earth in the morning sky, its phase increase but its disk size diminishes. Image credit: Statis Kalyvis

Although Venus was actually closer to Earth during the last month or so, the illuminated portion of Venus’ very thin phase also covered less square area of sky than it does at present. On December 1 or 2, 2018, (depending upon your time zone) Venus will reach its greatest illuminated extent – at which time the illuminated portion of Venus’ disk will cover the maximum amount of sky. At or near greatest illuminated extent, Venus always shines at her brightest in Earth’s sky.

Earth's and Venus' orbits

The Earth and Venus orbit the sun counterclockwise as seen to the north of the solar system plane. When Venus is to the east (left) of the Earth-sun line, we see Venus as an evening “star” in the west after sunset. After Venus reaches its inferior conjunction, Venus then moves to the west (right) of the Earth- sun line, appearing as a morning “star” in the east before sunrise.

Look carefully at the diagram above. Venus last passed between the Earth and sun at inferior conjunction on October 26, 2018. Some 72 days before and after inferior conjunction, Venus reaches its greatest elongation (maximum angular separation from the sun), at which juncture Venus’ disk is approximately 50% illuminated in sunshine.

At greatest eastern elongation, Venus reigns over the evening sky; and at greatest western elongation, Venus lords over the morning sky. In ancient times, the Greeks actually called Venus Hesperus when she dominated the evening sky and Phosphorus when she loomed over the morning sky.

Midway between greatest eastern (evening) elongation and inferior conjunction, Venus’ disk is about 25% illuminated in sunshine and this is when Venus shines brightest in the evening sky.

Then, midway between inferior conjunction and greatest western (morning) elongation, Venus’ disk is again 25% illuminated in sunshine and this is when Venus beams most mightily in the morning sky.

Watch the great celestial drama in the predawn/dawn sky, as Venus enjoys her moment of glory as the morning “star” in late November and early December 2018.

Bruce McClure

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