The celestial highlight in the coming days will involve the two brightest planets, Venus and Jupiter — but only one of the two characters in our celestial show has been on stage thus far. That is Venus, which has been a dazzling morning object since the beginning of April.
Meanwhile, Jupiter, which was a prominent evening object through much of the spring and summer, disappeared into the sunset fires during the final week of September and has been out of sight ever since. It passed through conjunction with the sun on Oct. 26 and has since been a morning object, but too close to the sun's glare to be seen. [The 2017 Venus-Jupiter Conjunction: When, Where and How to See It]
That will come to an end early on Saturday morning (Nov. 11), when this largest of the planets finally appears very low to the east-southeast horizon, about 45 minutes before sunrise and just 2 degrees directly below Venus. In the days to come, Jupiter will be slowly ascending, getting higher and more prominent, while Venus continues what has been a slow slide down toward the sun since mid-July.
They are like two ships passing each other in the dawn twilight.
On Sunday morning (Nov. 12), the apparent distance between these two worlds will be more than halved, and they will be separated by less than 1 degree. And on Monday morning (Nov. 13), they will appear closest together and rising in tandem, side by side: Venus on the left and dimmer Jupiter on the right. From the eastern U.S., they will be separated by a scant 17 arc minutes (0.28 degrees) — the equivalent of less than three-fifths of the apparent width of the moon. From the western U.S., when they rise, the gap between the planets will have widened slightly, to 21 arc minutes (0.35 degrees). Most telescopes will show them in the same field of view; the great Jovian planet will appear about three times wider than the nearly full disk of Venus.
Of course, what we'll be seeing is an illusion of perspective. The two planets are nowhere near each other; they just happen to line up when observed from our Earthly vantage point. Venus will be 152 million miles (246 million kilometers) from us, while Jupiter is nearly four times farther away, at 594 million miles (956 million km).
Weather permitting, for those observers who don't have any tall obstructions such as trees or buildings toward the east-northeast, this "double planet" should make for a very striking visual spectacle, no doubt attracting the attention of even those who don't give more than a casual glance at the sky. Venus will be markedly brighter than Jupiter; it will be at magnitude minus 3.9 compared to Jupiter's magnitude minus 1.7. Or, put another way, Jupiter will shine only about one-eighth as brightly as Venus. I suspect the combination of their great brightness, their closeness to each other and their low altitude will induce more than a few people to call local media outlets and maybe even police stations to report a UFO sighting.
By Tuesday (Nov. 14), Jupiter will have pulled away, appearing more than a degree to the upper right of Venus.
Conjunctions between Venus and Jupiter are far from rare events, taking place at mean intervals of 13 months — or, more precisely, every 398.88 days.
But what makes this week's Venus-Jupiter conjunction rather special is how unusually close they will approach each other. Using a computer program, I've found that over the next 100 years, up until the year 2117, Venus and Jupiter will come within 21 arc minutes or less of each other just 24 times. In fact, most of these events will occur during the daytime; only two will occur during twilight or in a dark sky. Monday's event is one of them, and the other is 22 years from now, on Nov. 2, 2039.
The 24-year cycle
Interestingly, there is also a 24-year cycle for Venus-Jupiter conjunctions. Their sidereal revolution periods — "sidereal" referring to "with respect to the stars" — are 224.7008 days for Venus, 365.2564 days for Earth and 4332.5894 days for Jupiter. As it turns out, 39 revolutions of Venus are almost precisely equal to 24 revolutions of Earth and two revolutions for Jupiter. So after 24 years, the circumstances of a particular Venus-Jupiter conjunction will appear to repeat under almost identical conditions.
|1921 Oct. 25||0.47 degree|
|1945 Oct. 30||0.47 degree|
|1969 Nov. 4||0.43 degree|
|1993 Nov. 8||0.37 degree|
|2017 Nov. 13||0.27 degree|
But such 24-year cycles cannot go on forever. Eventually, the two planets must break off and go their separate ways. Interestingly, two 24-year cycles from now, on Nov. 22, 2065, Venus will actually appear to cross in front of Jupiter! Too bad that the two planets will be only 8 degrees from the sun when this exceedingly rare event takes place, robbing us of an amazing visual spectacle.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmer's Almanac and other publications, and he is also an on-camera meteorologist for Verizon Fios1 News, based in Rye Brook, New York. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.