Should we complain? Two Great Comets have garnished Southern Hemisphere skies within the past 8 years – Comet McNaught in 2007 and Comet Lovejoy in 2011 – while a generation of northerners have had just photos. We are now treated to a near constant barrage of magnificent comet photos, but be aware that most are from highly experienced amateur astrophotographers using telescopes and solid-state sensors, or from professional astronomers using big telescopes, or even from the International Space Station, above Earth’s obscuring atmosphere. Meanwhile, from the ground and with the eye alone? Not since Comet Hale-Bopp in 1996-97 has the Northern Hemisphere seen a magnificent comet. What’s more, some skygazers would not classify Hale-Bopp as a Great Comet. In that case, we northerners have to look back to Comet West in 1976 – nearly 40 years ago – to find a Great Comet.
Let’s consider some of the incredible comets of recent times and historic records, to find out when the Northern and Southern Hemispheres might expect to see the next Great Comet.
First, how are we defining a Great Comet? There’s no official definition. The label Great Comet stems from some combination of a comet’s brightness, longevity and breadth across the sky.
For purposes of this article, to consider the question of Great Comets of the north and south, and their frequency, we’ll define Great Comets as those that achieve a brightness equal to the brightest planet Venus (magnitude -3 to -4) or brighter with tails that span 30 degrees or more of sky.
We can consider some other major comets, too, those that reached magnitude 1 or brighter – in other words, they became as bright as the brightest stars – with tails spanning 15 degrees or more. These major comets would have been visible long enough for Earth’s citizenry to take notice (some impressive comets have such extreme orbits that they aren’t visible long, and hardly anyone besides astronomers notices them).
Consider, also, that humanity’s ability to view the heavens has completely changed in the last 50 years.
In that time, space travel has become a reality and solid-state electronics have revolutionized photography. Space probes have been sent to comets beginning with the European Space Agency (ESA) Giotto spacecraft sweeping past Halley’s Comet in 1986, and, most recently, ESA’s Rosetta spacecraft … which is currently spending many months orbiting and becoming intimately acquainted with 67P/Churyumov–Gerasimenko.
And the transistor and sensitive solid-state detectors revolutionized astrophotography providing amateurs with observing capabilities far exceeding professionals prior to modern electronics.
The years 1996-1997 were all about Hale-Bopp for comet fans. It was primarily a Northern Hemisphere comet. For week on end, Hale-Bopp was a fixture in our western sky, and it probably became one of the most-viewed comets in history.
This comet was indeed a major comet, but a Great Comet?
Nearly all comets have short periods of visibility. Hale-Bopp literally smashed the previous record for longevity in our skies, which had been held for nearly two centuries by the Great Comet of 1811. The 1811 comet remained visible to the unaided eye for 9 months. Hale-Bopp was visible for an historic 18 months, truly, the Cal Ripken Jr. of comets.
Hale-Bopp was bright early on, nearly but not quite as bright as Venus. The size of its nucleus – the icy core of the comet, hurtling through space – was estimated to be 60 kilometers +/- 20 km (37 miles +/- 12). That makes Hale-Bopp’s nucleus some six times larger than the nucleus of Halley’s Comet and 20 times that of Rosetta’s comet, 67P/Churyumov–Gerasimenko.
Hale-Bopp had a long tail, up to 30 degrees long, but what was visible and bright was relatively a short tail, less than 10 degrees long, for nearly its entire period of visibility. Yes, some former Great Comets did not have 30-degree-or-longer tails, but those comets were, instead, extremely bright.
Bright generally means as bright as Venus or brighter. Hale-Bopp was not quite that bright. Some great comets are visible in daylight, but Hale-Bopp was not.
Finally, probably, we have to concede that Hale-Bopp straddles the edge of greatness.
In 1973, skygazers were alerted to the early discovery of a comet called Kohoutek. At the distance at which it was discovered and its brightness, astronomers projected that this was going to be a Comet of the Century, perhaps a daylight comet, a once-in-a-lifetime event.
But Kohoutek fizzled. It really disappointed skygazers even though, for professional astronomers, the drawn-out observations of Kohoutek were quite valuable.
Astronomers thought they had learned a lesson from Kohoutek. Too many astronomers stood outdoors at public “star parties” that year, trying to show a disappointed public a difficult-to-see comet.
Unfortuntely, the lesson learned from this comet led to astronomers to downplay the next contender for greatness – Comet West in 1976. That was too bad, because Comet West did not disappoint. It was a magnificent comet! Still, many average skygazers were left out because astronomers remained quiet and the media did not report. Comet West was not seen and appreciated as it should have been.
From Comet West, fast forward a full 31 years to 2007 and the next truly Great Comet (sidestepping Hale-Bopp). The comet hunter Robert H. McNaught – who has discovered more than 50 comets – discovered it. This 2007 comet is sometimes called the Great Comet of 2007. You’re in the Northern Hemisphere and don’t remember a Great Comet that year? That’s because, due to the inclination and high eccentricity of comet orbits, many are viewable from only one Earth hemisphere or the other. That was the case for Comet McNaught in 2007.
Only Southern Hemisphere skygazers had a chance to become enamored of Comet McNaught in 2007. Then, just four years later, another Great Comet appeared in Southern Hemisphere skies, Comet Lovejoy of 2011. Northerners could only watch these two comets from a distance, through the wizardry of the digital age. Or they could hitch a costly ride to place themselves under the southern skies.
So now consider the following chart which plots the major and Great Comets going back to 1680. Bear in mind that astronomical records appear to have reached a high level of fidelity about 200 years ago. Looking at this data statistically, what does it reveal?
On average, every 5 years, one can expect to see a major comet visible from the Earth. However, the variability around that average is also about 5 years (one standard deviation).
This means that, on average, a major comet arrives every 5 to 10 years.
Sometimes the visitations are clustered. A prime example is the years 1910 and 1911, when four major comets crossed the sky.
The data also reveal that Great Comets arrive on average every 20 years. The variability is 10 years, as represented by a standard deviation around the average. So truly Great Comets may be visible from Earth every 20 to 30 years. Some centuries might have two or three (1800s) while others, four or more (1900s).
Statistically, accounting for comet activity over 250 years – 38 major comets – is pretty sparse data, but one can see in the plot an historic trend. It is possible that if data could reveal a leaning towards one hemisphere, it could be an indicator that the Oort Cloud, north or south of the ecliptic plane was affected by some object, e.g. a passing star. There is no indication of this in the records.
Does it answer the question – has the Northern Hemisphere missed out on Great comets?
There is certainly a recent trend towards the Southern Hemisphere for Great comets. The data reveal that the long-term trend for both the Southern and Northern Hemispheres is a Great Comet every 25 to 40 years.
But, if you discount Hale-Bopp, then the last Great Comet for the Northern Hemisphere was Comet West, 39 years ago. Even if you consider Hale-Bopp as “great,” 20 years have passed.
It would seem that the north is statistically ready to receive its next Great Comet. Bring it on!
Bottom line: The Southern Hemisphere has had two Great Comets recently – McNaught in 2007 and Lovejoy in 2011. But what about the Northern Hemisphere? Our last widely seen comet was Hale-Bopp in 1996-97. Comet West in 1976 was probably our last Great Comet.