Bring in winter with a bang by observing a beautiful, close conjunction and a rare planet-star occultation. 

Light to soften the night
Participants at a winter solstice celebration in Two Harbors, Minnesota, gather around a bonfire as they drum in the new season. Friday is the first day of winter.
Bob King

On Friday, December 21st, at 5:23 p.m. (Eastern Standard Time), the Sun arrives at its lowest point in the sky for the Northern Hemisphere, marking the winter solstice and the official start of winter. Ancient peoples built monuments like Stonehenge to celebrate this special date, positioning massive stones to align with the setting Sun. If these megalith makers could travel forward in time to this Friday, they might find special meaning in two noteworthy celestial alignments that occur only hours before the official start of the new season — an occultation of a 6th-magnitude star by Venus followed by a close conjunction of Jupiter and Mercury.

Venus rarely occults a star bright enough to see through the planet's signature glare. But on Friday morning the planet will pass directly in front of 5.9-magnitude HD 130325 (HIP 72373, SAO 158808) in Libra around 12h UT (7 a.m. EST). The event will be widely visible across the eastern two-thirds of the U.S. and Canada as well as Central America. To see it you can use almost any telescope, but make sure you use high magnification to separate the star from Venus's dazzle. The planet will be 41% illuminated at the time.

Viewing zone
The above map shows where the occultation will be visible, weather permitting.
Occult 4.5

Disappearance will occur at the planet's brilliant (eastern) limb and likely be impossible to see because of the dramatic difference in brightness between the two. The star will reappear only minutes later from behind the dark limb and should be relatively easy to spot because 18 arcseconds of dark, un-illuminated planet will stand between it and the Venusian glare. The limiting factor will be Venus's altitude and the local seeing conditions.

Eastern locations will see Venus highest at around 30°, but the occultation occurs in morning twilight, which may make spotting the star challenging. In the Midwest, the planet stands about 20° high in a dark sky, making this region an ideal location for viewing the event. For viewers in the Mountain Time Zone, Venus will climb to around 10°. West Coast observers will miss the occultation with Venus still be below the horizon.

This example shows the occultation circumstances early Friday morning for skywatchers in the Chicago area. The star's reappearance should be visible under good seeing conditions.
Stellarium with additions by the author

Because Venus has a thick atmosphere the star should emerge rather more slowly compared to the sudden, on-off occultations by the airless Moon. David Dunham, a founder of the International Occultation Timing Association (IOTA), points out that those with large telescopes can possibly observe a unique phenomenon caused by refraction of the star's light by the planet's atmosphere:

". . . those with large telescopes will probably see the star faintly a few minutes before the predicted reappearance, due to refraction of the star’s light. In fact, due to Venus’s thick atmosphere, the star will never completely disappear, but will appear as a spot, at about 12th magnitude at its faintest, on the edge of Venus at the point on its disk closest to the star." View the video below to see this in action with Titan.

Above: Titan occults a binary star captured in this time lapse made with the 200-inch Hale Telescope on Mt. Palomar. Download the full-resolution video here.

In an e-mail communication, Dunham wrote that recording the rare lensing event in photos and video may be impossible due to glare from Venus, but it might be seen visually as a fainter mimic of the star whipping around the planet's limb during the occultation. We have just one recording showing a similar event. On December 20, 2001, Titan covered both components of a double star separated by just 1.5″. Watch the video linked above to see the refracted image pop up on upper limb of the Moon during the first occultation and along the bottom during the second. Truly an amazing sight!

Rare flash visibility zone
Observers along the occultation's centerline may catch sight of the central flash. 
Google Maps / David Dunham

Observers along the occultation's central line, which passes from Wyoming to South Carolina, might see an equally rare central flash, where the atmosphere of Venus focuses the star's light into a bright ring outlining the dark limb. Dunham estimates that it should be visible from a strip 500 meters wide with a maximum duration of 0.03 second, but there should be at least a few seconds ramp-up to the maximum that would also give some central flash visibility over a range of perhaps 20 kilometers.

Above: Watch for the central flash during Triton's occultation of a star on October 5, 2017. Because Triton's disk isn't resolved, the entire moon brightens. Venus has a measurable disk, so the flash will appear as a bright circle around the planet's limb instead.

With so many fascinating aspects to this occultation I hope you'll set your clocks, find a location with a good view of the southeastern horizon, and have a go at it. Be sure to allow a half-hour of cool-down time for your telescope, so your view of Venus will be as sharp as possible. The following links will help you in preparations:

Dawn planet panoply
Look for a lively dawn tableaux Friday about 45 minutes to an hour before sunrise with a close conjunction of Mercury and Jupiter along with Venus, fresh off its occultation an hour earlier.

Now for dessert. When you're finished with the occultation, have an early breakfast and then catch a wonderful pairing of Jupiter and Mercury. Jupiter's just returning to the dawn sky, and finds quick company with the fleet planet. They'll pair up in Ophiuchus low in the southeastern sky two-and-a-half fists (25°) to the lower left of Venus. Jupiter will be the obvious one, shining at magnitude –1.75 with Mercury no slouch at –0.5 magnitude. Because they're separated by just 53 arcminutes you can probably squeeze both in the same low-power field of view of your telescope. Mercury will be 77% illuminated and look like a tiny version of the waning gibbous Moon. If bad weather intervenes, the two will still be close on Saturday the 22nd, just a hair more than a degree apart.

I think you'll agree — December's been a busy month for skygazing!


Mercury Venus


Image of Rod


December 19, 2018 at 10:59 am

I was looking forward to viewing this event with Venus and the star on the 21st but now it looks like rain coming Thursday night through Friday in Maryland where I view. Some folks will likely get very good views. On 18-Dec-18 near 0600 EST I enjoyed viewing Venus and a star very close to Venus in the field of view (FoV) at 56x magnification. Venus was in Libra. "Observing Venus, HIP71469 star < 0.5 deg away in 9:00 position, mirror reverse image. The star is nearly 12.5 million times farther away from Earth than Venus at 0.533 AU distance this morning. Good position fix and view of Earth's rotation rate...Venus mv -4.70, HIP71469 mv +6.2 so Venus was about 22920x brighter than the star using apparent magnitude scale". Folks can have fun viewing Venus and other planets as they travel along the ecliptic and pass various stars in the field of view using small telescopes or even larger telescopes.

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Image of Bob King

Bob King

December 19, 2018 at 2:46 pm

The movement of the planets makes for endless instruction and entertainment, not to mention art.

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Image of Jakob


December 24, 2018 at 4:04 am

A cold and clear Christmas morning, Mercury, Jupiter and Venus were caught in action. One of the best presents this holiday. Only two days of observering in an otherwise cloudy december. From a rural sky on the 10th I glimpsed our visiting comet as well. Merry Christmas from the country of low pressure wheather. 🙂

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Image of Rod


December 24, 2018 at 8:48 am

Jakob, Merry Christmas too. I am in Maryland and did not get to see the Venus occultation in this report, clouds and rain that morning but skies clear now and looking better. This morning I enjoyed viewing Venus using my telescope at 56x and with a moon filter (worked well) :). The double star Zubenelgenubi or Alpha Librae in Libra was visible in binocular view being a bit more than 3 degrees from Venus. Easy double star split in 10x50 binocular view. This was near 0600 EST or 1100 UT on 24-Dec-18. The waning gibbous Moon was in Gemini near 29 degrees altitude and 273 degrees azimuth while Venus close to 26 degrees altitude and 137/138 degrees azimuth. This was a lovely view early this morning along the ecliptic in SE sky with Venus, Zubenelgenubi double star, and W sky with the waning gibbous Moon.

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