Say Goodbye To Venus

Check out this triangle of lights this evening shortly after sunset in the western sky. FYI, tomorrow night (Sept. 13), the moon will be in conjunction with Jupiter, passing 3.5° above the planet. Stellarium

Look to the west at dusk this evening and give Venus a goodbye kiss. If you haven’t noticed already, Venus has been practically MIA these recent weeks as it tracks back toward the sun at dusk. For now, the planet stands only about 4° high 30 minutes after sunset from my northern location. To spot it, observers in the northern U.S. and Canada have to find a wide open horizon and be on time — it’s only visible for a half-hour or so beginning shortly after sunset.

Happily, the further south you live, the higher the planet stands and the longer it will be in view. So what gives? Conjunction with the sun doesn’t occur until October 25, so why is the planet so low right now even though today it’s 42° from the sun — just a few degrees shy of it greatest apparent distance? It has to do with the angle the ecliptic, the path followed by the moon, sun and planets around the sky.

The angle the ecliptic makes to the western horizon has all to do with the visibility of Venus. In late summer and fall, it’s shallow and the planet sets soon after the sun. In spring, its stands high in the west after sunset. Venus’s orbit is also inclined 3.2° to the plane of Earth’s orbit (which defines the ecliptic), so it sometimes dips above and below it. Right now, it’s below the ecliptic, another reason why it appears so low at dusk. Stellarium with additions by the author

Because of our planet’s tilted axis and changing seasonal position as it sweeps around the sun each year, the angle the ecliptic makes to the western horizon also changes. In spring, that angle is almost vertical to the western horizon at dusk. When Venus is 42° from the sun in March and April, nearly all of those degrees are straight up and away from the sun, so it appears high in the western sky after sunset. In early fall, the ecliptic makes a much shallower angle to the horizon at dusk. It practically lies down flat especially for northern skywatchers. Most of that 42° is horizontal distance with only a few in the vertical, so the planet shines very low in the hazy murk.

While it’s just not fair, it is what it is, which is why Venus-watchers have to work a little harder on fall evenings than spring ones. (By the way, the ecliptic situation is reversed in the morning sky with Venus much higher up in the east in early fall and lower in spring.) No worries, the moon’s got you covered. Tonight, a beautiful 3-day-old crescent floats 8.5° or just shy of one fist above the planet and two fists to the right of Jupiter. If you’re out starting about 20 minutes after sunset (sunset times here), find the moon and then look almost directly below it about three fingers held together at arm’s length for bright Venus. Yes, at least Venus is still the brightest thing around besides the moon.

As Venus orbits the sun every 225 days it appears to move from one side of the sun to the other and changes phase from our perspective. When east of the sun, it’s the “evening star.” When west of the sun, it comes up before sunrise as the “morning star.” Bob King 

I encourage you to bring along your binoculars. The planet’s now in its crescent phase, about as thick as a banana, and it’s shape is just discernible in a 10x glass. Venus will continue to thin and enlarge in the weeks to come as it heads toward inferior conjunction, when the faster-moving planet passes between the Earth and sun. After Oct. 25, it swings to the other side of the sun and will reappear in the morning sky before sunrise in early November.

Just a note about the aurora the past few nights. As often happens, the storm arrived but not quite at the time predicted. While you could see a minor display Monday night, it only reached the G2 or moderate storm level around 3:30 Tuesday morning. Last night, I looked for signs of a minor storm between 10 and midnight but saw no trace. The long-range forecast calls for quiet magnetic conditions for the next few weeks.

5 Responses

  1. caralex

    Had a lovely clear evening on Sunday, so just after dusk, it was easy to see Venus, Jupiter, Saturn and Mars spread out over the south-facing sky!

    1. astrobob

      Interesting story, Jason. Thank you for the link. Could be anything: chemical leak, threat against someone’s life, terrorist plot or even drug-related. I’d say the least likely possibility is aliens. It’s usually humans causing all the trouble.

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