Sky Notes for July 2012, with James Abbott #stargazing
Summer twilight lasts through most of July although so far this summer clear skies have been at a premium so there have only been a few chances to observe the deep blue midnight sky to the North. In the gaps in the weather, noctilucent clouds have been observed to be active with several displays seen from Essex.
By the last week of the month the summer twilight period is coming to an end and it is fully dark again in the small hours.
Full Moon is on the night of July 3rd/4th, so the best dark skies for will be from about the 9th July until the last few days of the month when the Moon is once again waxing.
Looking East in mid to late July, the summer constellations are high up with the brilliant blue/white Vega almost overhead. Away from streetlights the Milky Way should be fairly easy to see, stretching from high in the East down to the South. By midnight its a case of “three seasons in a night” as the autumn constellations of Pegasus and Perseus are rising in the East, with the bright yellowish star Capella, a star associated with winter, low in the North.
A variety of meteor showers are active through July and August so that on any clear Moonless night it should be possible to see a few meteors even in a watch of half an hour. Almost all of these regular annual showers are the product of comets leaving trails of dust behind which the Earth then passes through at the same point in our orbit each year. So to see a meteor is usually to see a tiny fragment of a comet burning up high in our atmosphere. Regular showers in July originate from radiants in Cygnus, Capricornus, Aquarius and by late July the most famous shower is becoming active, the Perseids. But most of these summer showers are individually weak, so working out which shower an apparently lone meteor is a member of, or if it is a sporadic meteor, can be tricky from visual observations alone. Tracing the track back across the sky can point to the possible source constellation, but an additional check is the velocity across the sky. Alpha Capricornids for example can flare brightly and are noticeably slow, with an entry velocity of only 25km/sec, which is less than half that of Perseids.
Mars and Saturn continue to get closer in the sky as the month progresses and by the end of July are just 8 degrees apart, and along with the star Spica, will make up a triangle of 3 objects all of similar brightness. However, this will be low in the West soon after dusk. The young Moon will pass about 5 degrees below Mars on the evening of the 24th.
In the early hours of the 15th July, Jupiter will be occulted by the waning crescent Moon. With Essex close to the southern graze line, observers in mid-Essex will see Jupiter just covered by the Northern limb of the Moon just after 3am BST. To add to the spectacle, all 4 Galilean moons will also be occulted. The whole event only lasts for about 30 minutes, so observers with telescopes will need to be set up ready by 2.45am BST. To add to the attraction, Venus will be just 6 degrees from Jupiter and the Moon. A good clear North Eastern horizon will be needed as the altitude of Jupiter is just 10 degrees and Venus only 4 degrees at 3am BST.
James Abbott is an astronomer, NEAS member and CfDS Regional Information Officer.