★ Southern Delta Aquariid Meteor Shower

The Southern Delta Aquariids are the main shower of late July and one of the most active of the year in the Southern Hemisphere. From mid-northern latitudes the radiant stays fairly low in the south, so you see fewer, but on a dark night at the end of July they still add a steady stream of medium-speed meteors to the early Perseids.

Where the meteors come from

The parent is thought to be Comet 96P/Machholz, which Donald Machholz discovered in 1986; NASA calls the link suspected rather than proven. The comet's nucleus is about 6 km across, and it orbits the Sun about every 5.3 years on a steep, elongated path that brings it within about 19 million km of the Sun, much closer than Mercury. Machholz belongs to a family of related objects called the Machholz complex, and the IAU Meteor Data Center also lists two small sun-skimming comets from it, 342P/SOHO and 2008 Y12, as possible sources. The grains hit the atmosphere at about 41 km/s.

When to watch

The shower runs from July 12 to August 23, peaking around July 30–31. Rates stay near their best for about two days and not far below that for about five nights. The table above gives the expected peak and the moonlight. The radiant rises in the late evening and is highest around 2 a.m. local standard time (3 a.m. on summer time), so the hours from midnight to dawn twilight are best.

How to find the radiant

The radiant is in southern Aquarius, about 3 degrees from the star Delta Aquarii (Skat), which gives the shower its name. Aquarius is faint, so use Fomalhaut, the lone bright star low in the south in the early morning hours. The radiant is about 14 degrees north of it, a fist and a half at arm's length; from northern latitudes, that is above Fomalhaut and slightly to the right. From about 40°N the radiant climbs to only about 34 degrees. From the Southern Hemisphere it passes high overhead, which is why the shower is so much richer there.

What to expect

The ZHR is about 25. From around 40°N, with the radiant no higher than 34 degrees, you see a bit over half of that even under a perfect sky: a dark rural site might give 8 to 12 an hour at best, the suburbs 3 to 5, in line with NASA's estimate of 7 to 8 under dark skies. From mid-southern latitudes, 15 to 20 an hour from a dark site is realistic. The meteors are of medium speed, slower than the Perseids. Around the peak the population index is 2.5, so a fair share are bright; away from it, it rises to about 3.1 and most are faint.

Viewing tips

• Find a site with a dark southern horizon. From the north the radiant is low, and town lights to the south cost you meteors.
• Watch between midnight and dawn twilight, facing south or southeast and looking about halfway up.
• Make it a double session. Southern Delta Aquariids come from the south and faster Perseids from the northeast; slow meteors from the same southern area may be Alpha Capricornids.
• If the peak night is cloudy or moonlit, try the nights before and after.
• Late July is winter in the Southern Hemisphere, so southern observers should dress for long, cold nights.

History and notable displays

The Southern Delta Aquariids are a long-established shower, number 5 in the IAU list. Rare outbursts with a ZHR of about 40 were reported by Australian observers on 1977 July 28/29 and from Crete on 2003 July 28/29, both a little before the date of maximum found in recent years. The radiant shares its part of the sky with the Antihelion source, a broad, weak radiant active most of the year; the IMO keeps the two apart because Southern Delta Aquariids are faster. It asks observers to keep monitoring the shower's activity level and variations.