As of Friday, December 8, we can finally say with absolute certainty that Halley's Comet is coming.
This most famous of all comets travels in a flattened elliptical orbit around the Sun, bringing it close to the Sun and then taking it far beyond the boundaries of the Solar System. Since reaching perihelion - the comet's closest approach to the Sun - on February 9, 1986, it began its long journey back into deep space. And from that moment until now, the comet has been moving inexorably away from the sun.
But at 8:00 PM Eastern Time on Friday, December 8 (1:00 AM GMT on December 9), that will come to an end. For at that time Halley's Comet will reach aphelion; the end of its orbit - that location in space that places the comet at its farthest point from the Sun: 3.27 billion miles (5.26 billion km) away. The comet will then be 472.2 million miles (759.8 million km) beyond the orbit of Neptune, the farthest known planet.
The last time Halley was at this point in his orbit was in April 1948.
Related: The Eta Aquarid meteor shower is the legacy of Halley's Comet in the May night sky
When Halley reaches aphelion, its orbital velocity will have slowed to just 0.909 km per second; approximately 2,034 miles (3,272 km) per hour. Compare that to when the comet orbited the Sun at perihelion at 54.52 km per second on February 9, 1986; approximately 121,572 miles (195,609 km) per hour.
So after December 8, the comet will approach the sun again - for the first time in almost 38 years. In accordance with Kepler's second law of motion, a celestial body moves fastest when it is at perihelion and slowest at aphelion. So once Halley passes aphelion, its orbital speed will begin to increase-very slowly at first-on its way toward the Sun.
In the table below we note the times when Halley will cross the orbits of seven planets as it moves towards the Sun. The average distance of each orbit is given in astronomical units (AU). One astronomical unit is equal to the average distance from Earth to the Sun of 92,955,807 miles (149,597,870 km).
Halley's Comet will re-enter perihelion on July 28, 2061.
Looking ahead to 2061
So, what can we expect from Halley's next performance in 38 years? With the current average life expectancy for Americans at almost 79 years, if you were born after 1982 you have a better than 50 to 50 chance of witnessing Halley's return in the summer of 2061.
Moving toward the Sun, it will be visible in the morning sky during late spring and early summer, favoring viewers in the Northern Hemisphere.
Interestingly, 2061 is a mirror image of the comet's last appearance in the winter/early spring of 1986. At the time, the comet was out of sight and on the far side of the Sun in midwinter; but by mid-summer 2061 the comet will be on the same side of the sun as us, in full view, and appear at least ten times brighter! It will then drop rapidly, gradually favoring more southerly locations as it enters the evening sky.
But for viewers in mid-northern latitudes, the comet should still evolve into a standout, albeit low in the west-northwest sky on early August evenings.
A two-month parade
The main show starts in mid-June in the morning sky. On June 18, Halley's Comet will be in the constellation Taurus, 1.2 degrees northwest of the Pleiades. It will be quite faint around magnitude +5.6 and observers in good locations are unlikely to see the bluish ion tail much longer than 1 degree; an artifact of its distance from Earth of 167 million miles (269 million km).
But the comet will approach both the Sun and Earth with increasing speed, and so each morning it will appear as if it is climbing higher in a darker sky, becoming noticeably brighter.
On July 1, it brightened to magnitude +4.3, with a short tail possibly a few degrees long. On July 10, it will be 93 million miles from Earth and its brightness will have more than doubled to magnitude +3.5. A week later, now around magnitude 2.5, the tail should be more visible and reach a length of about 5 degrees; Halley races from east to northeast through the pentagon of the constellation Auriga.
On July 23, the comet's head may now be as bright as first magnitude, low in the northeastern morning sky, with a short tail pointing almost straight up from the horizon.
Meanwhile, Halley will gradually transition from the morning to the evening sky, and as it appears to track about 21 degrees north of the Sun at perihelion, from July 25 to 28, it will be possible to see it in the late twilight sky. as both a zero-magnitude morning and evening object. Now its bright, white dust tail, which greatly impressed our ancestors, begins to unfold.
Evening show... Halley at his best!
As August begins, the comet becomes exclusively an evening object. Initially its visibility will be obscured by the presence of a full moon on August 1, but by the evening of August 4 the comet will finally shine unobstructed in all its glory in a dark sky low in the west as darkness falls . .
August 4 through 8 may be the peak of the 2061 appearance: the comet's head, shining as bright as first magnitude, is now accompanied by a straight, narrow tail that tilts perhaps 10 to 15 degrees flows out.
In the evenings that follow, as Halley withdraws from both the sun and the earth, its luster will dim. Furthermore, as it passes southeast through the stars of Virgo, it will fall lower towards the horizon: magnitude +1.8 on December 10, after which skywatchers will be in for a bonus on the evening of December 18 when it forms an isosceles triangle forms with an almost four-day old crescent moon and Venus; it has now dimmed to magnitude +2.8. Halley next passes within one degree of Venus on December 24, having faded to magnitude 3.3 by then.
Light pollution is the wild card
RELATED STORIES:
- Halley's Comet: Facts about history's most famous comet
- Capturing Halley's Comet: An Astronomy Story
- Halley's Comet was spotted by the ancient Greeks
With the average life expectancy for Americans now approaching 79 years, if you were born after 1982 you have a better than 50 to 50 chance of witnessing Halley's return in the summer of 2061.
However, it is my fervent hope that in the next 38 years we will somehow be able to find a solution to significantly stem the growing tide of light pollution.
If not, there is a very real danger of us being 'electrically bulldozed' and the view of Halley being blotted out by bright lights. And as such, our children and grandchildren may well be denied their rightful opportunity to greet Halley themselves.
In short, while we can give a pretty good estimate of what Halley's Comet will do on its next scheduled visit, in terms of predicting the state of our night sky by then, it's anyone's guess.
Joe Rao is an instructor and guest lecturer in New York Hayden Planetarium. He prescribes on astronomy Natural history magazinethe Farmer's almanac and other publications.
