Tonight – February 16, 2017 – according to clocks in the Western Hemisphere, Venus reaches its greatest illuminated extent, at which juncture Venus is at or near its greatest brilliancy in Earth’s evening sky. Are you in the world’s Eastern Hemisphere (Australia, New Zealand, Asia)? If so, this same event – greatest illuminated extent – takes place for you on February 17.
If you look for Venus in the west after sunset, you might notice another planet near it. That planet is Mars:
Greatest illuminated extent means Venus’ day side, or illuminated side, is covering more square area of Earth’s sky than at any other time during this current evening apparition of Venus. And that means that Venus is brighter around now than at any other time during its approximate 9.6-month reign in the evening sky.
Surprisingly, perhaps, Venus is actually at its brightest on February 18, about one and one-third days after reaching its greatest illuminated extent. Our friend, Guy Ottewell, explains why the two events don’t quite coincide in his recent post on Valentine’s Day.
Plus Mars is still near Venus in the west after sunset. Amazing evening sky! Watch for these two worlds.
Why is Venus so bright now? You might think Venus appears most brilliant when we see its disk as most fully illuminated from Earth. Not so. If you were to observe Venus with the telescope at its greatest illuminated extent, you’d see that Venus’ disk is only a touch more than one-quarter illuminated by sunshine.
A full Venus is always on the far side of the sun from us, so its disk size at full phase is always small.
It’s only when we see Venus as a crescent that this world comes close enough to us to exhibit its greatest illuminated extent, at which time its daytime side covers the greatest area of sky.
The time of greatest illuminated extent for Venus – for Earth as a whole – is 07:00 UTC. Converting Universal Time to what our clock time in U.S. time zones, Venus is at its greatest illuminated extent on February 17, at 2 a.m. EST, 1 a.m. CST, 12 midnight MST and at 11 p.m. PST (on February 16).
Interesting to know an exact time … but no matter where you live on Earth, Venus appears in your western evening sky after sunset.
Venus last transitioned from the morning to evening sky when it swung directly behind the sun from Earth (superior conjunction) on June 6, 2016. Venus reached its greatest eastern (evening) elongation from the sun on January 12, 2017. It will swing between the Earth and sun (inferior conjunction) on March 25 and thereby enter the morning sky. It will reach its greatest western (morning) elongation from the sun on June 3, 2017.
Venus won’t pass precisely between us and the sun on March 25. If it did, Venus would transit the sun, as it did in June of 2012. This time, Venus will swing 8o north of the sun. That means sky gazers in the Northern Hemisphere have a good shot at viewing Venus in both the evening and morning sky for several days around Venus March 25 inferior conjunction!
Venus exhibits its greatest illuminated extent about 36 days before – and after – inferior conjunction. Through the telescope, Venus appears about 25% illuminated in sunshine at these times.
Let the golden triangle help you to remember these Venus’ milestones. The two base angles equal 72o and the apex angle equals 36o. Quite by coincidence, Venus’ greatest elongations happen 72 days before and after inferior conjunction, and Venus’ greatest illuminated extent happens 36 days before and after inferior conjunction. (See above diagram of Venus’ and Earth’s orbits.)
Bottom line: Enjoy Venus at greatest brilliancy around February 16-17, 2017. Even though this world is only about one-quarter illuminated in sunshine right now, as seen from Earth, Venus is nonetheless shining at its brightest best in the evening sky! Plus, Mars is nearby. Both can be found in the west after sunset.