Bring in winter with a bang by observing a beautiful, close conjunction and a rare planet-star occultation.
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.
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.
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!
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:
- List of cities with disappearance and reappearance times The predictions are ordered alphabetically by the country 2-letter code (CA for Canada, MX for Mexico, and US for the United States). The times are Universal time (UT); 12h UT is 7 a.m. EST, 6 a.m. CST, 5 a.m. MST, and 4 a.m. PST.
- Interactive Google Map where you can zoom in on the center line to scout a "central flash" location.
- Complete occultation information
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!