On any clear night, if you look at the sky long enough, chances are you’ll see a meteor streak across the sky. However, some nights are better than others.
At certain times of the year, Earth passes through particularly dirty parts of its orbit, plowing through debris left behind by comets and asteroids. During those times we see debris crash into our atmosphere, and a meteor shower is born.
Single meteor shower is better than others. The faster the debris moves, or the more debris there is, the more meteors you will see. But generally, these showers are annual events – repeating as the Earth returns to the same place in its orbit.
The end of July is one such time, with the Earth passing through several parts of our solar system’s debris at once.
Two of those showers peak around July 31st. Although not one of the very best showers of the year, the two together can put on a lovely show in the depths of our cold winter nights.
Southern Delta Aquariids: the fast ones
The first, and most active, of the two showers is the Southern Delta Aquariids. For stargazers in Australia and New Zealand, they are the third strongest meteor shower of the year (after the wonderful Geminid, in Decemberand the Eta Aquariids, which peak in early May).
The Southern Delta Aquariids are dust from comet 96P/Machholz – a dirty snowball moving on a highly elongated and tilted track within the inner solar system. 96P/Machholz is the largest object in a broad stream of debris that produce several meteor showers throughout the year.
The Southern Delta Aquariids are active for about six weeks, from mid-July to late August, and reach their peak on July 31st. In a typical year, the shower is at its best for about 48 hours. During the peak, observers can see up to 20 to 25 meteors per hour under perfect conditions.
While many meteors from this shower are relatively faint (and thus become more difficult to see if the moon is above the horizon, or if you are observing from a light-polluted location), the shower is known to produce some brighter meteors, especially around their peak.
Additionally, the Southern Delta Aquariids have produced at least two unexpected eruptions in the past, with increased rates observed in 1977 and 2003 – a reminder that meteor showers can sometimes produce nice surprises!
Alpha Capricorns: slow, with occasional fireballs
The Alpha Capricornids are a significantly weaker shower than the Southern Delta Aquariids – they produce fewer meteors per hour. Even at their best, on the nights of July 30 and 31, observers rarely see more than four or five meteors from the shower in any given hour.
But where the Southern Delta Aquariids are abundant, fast and often faint, the Alpha Capricornids are slow, and often bright. Indeed, the shower has a reputation for producing spectacularly bright meteors and fireballs. Its meteors, rare as they are, are often the highlight of a winter night’s sighting.
In 2010, two of the world’s leading meteor scientists identified the parent of the Alpha Capricornid meteor shower – called a dim comet 169P/NEAT. They suggest that it is just a small piece of a larger object that fragmented between 4,500 and 5,000 years ago.
Currently, Earth is only passing through the very outermost layers of a large debris flow deposited by that ancient fragmentation. The scientists who identified it predict that in just 200-300 years we will move through the middle of the stream instead.
If that happens, the Alpha Capricornids will one day become by far the best meteor shower of the year.
Where and when should I look?
This year, the peak of both meteor showers falls midweek, on Wednesday 31 July. However, both showers have relatively broad peaks and will produce respectable numbers of meteors for several days.
If you’re planning a camping trip on the weekend of July 27–28 or August 3–4, you might still get a decent showing, especially in the wee hours after midnight.
But for the best rates, you should head out on the evenings of Tuesday 30 July and Wednesday 31 July.
From all over Australia and New Zealand, you can start observing from 21:00 or 22:00, when the rays for both showers – the place in the sky from which meteors appear to radiate – rise in the east. At first rates of the showers will be low, but the higher in the sky the beams rise, the more meteors will be visible.
The bright stars Altair and Fomalhaut are useful guides. As a bonus, the planet Saturn can be found in the same part of the sky, shining as brightly as the brightest stars.
The longer you’re willing to stay outside, the better your chance of seeing meteors. As the night progresses, the jets will move across the sky and climb higher until they culminate in the north after midnight. The best rates will be visible when the rays are at their highest: between about 11pm and 3am.
Go somewhere far away from city lights. Our eyes take a significant amount of time to adjust to the darkness, so it’s best to watch for at least half an hour, if not longer – especially since meteors are not evenly spaced. You can wait 20 minutes and see nothing, then see several in just a minute or two!
If you are lucky enough to find a place where the sky is dark in all directions, you should look to the north-east in the evening, to the north in the hours around midnight and then to the north-west in the pre-dawn hours.
The darker the sky, the more you will see. By the peak of the two showers, the moon will be almost out of the way and rise just a few hours before dawn.
As a result, this year is the ideal time to get out and watch an annual winter spectacle. And who knows, you might just get lucky and see a spectacular fireball caused by the debris dumped by a dying comet 5,000 years ago.
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Jonti Horner is a professor of astrophysics at the University of Southern Queensland. Tanya Hill is an Honorary Fellow at the University of Melbourne and Senior Curator (Astronomy) at Museums Victoria Research Institute. This article was originally published in the conversation
