The Night Sky in August
The annual Perseid meteor shower reaches peak activity on the night of Aug 12-13 and, if the weather is kind to us, this year’s display could be something to remember. Two things can conspire to ruin a meteor shower; the weather and the Moon. This year new Moon occurs on Aug 11 and this natural light polluter will be absent during the Perseid’s peak period.
Perseid meteors occur when our planet passes through the dust strewn around the orbit of comet 109P/swift-tuttle. Earth first enters this region around the middle of July when the dust density is low and just a few meteors are seen. The density continues to increase throughout July, but it’s not until we get towards the central part, around the end of the first week in August, that rates start to climb significantly.
The densest part is reached on the night of Aug 12-13. Earth then moves out through the other side of the stream and rates drop off once again. We fully exit the stream around the third week of August. The most impressive rates tend to occur in the week of Aug 9-16.
The activity of a meteor shower is given by its zenithal hourly rate, or ZHR. The Perseid shower has a ZHR of 80-100 meteors per hour at peak, and it is this figure you’ll most likely see bandied around. However, the ZHR can be misleading because it assumes optimal conditions. In reality few of these are ever truly met, and the number of meteors you can actually expect to see – the visual rate – will always be lower.
Under normal conditions from a dark sky location in the UK, the peak Perseid visual rate may achieve 30-40 meteors per hour, which is still respectable. The particles strewn around the orbit of comet 109P/swift-tuttle are effectively travelling in parallel paths. They enter our atmosphere and vaporise, creating a streak of light known as a meteor trail. Perspective makes it appear that the shower meteors spread out from a single area of the sky known as the shower radiant. The Perseid radiant slowly moves over time. Mid-july it sits south of the constellation of Cassiopeia. From there it heads east, through Perseus and into Camelopardalis. Peak activity occurs when it’s within Perseus, hence the shower’s name.
Watching Perseid meteors couldn’t be simpler. The best advice is to use a sunlounger or some other arrangement where you can lay back comfortably. Despite being summer, August nights can still feel damp, so remember to wrap up warm. Find a dark spot with no stray lights and give yourself 20 minutes in darkness for your eyes to properly dark adapt. Look at a height two thirds the way up the sky in any direction and wait.
If you want a recommendation, looking south before 1am gives you a good view of the Milky Way, assuming your sky is dark. Then in the early hours, turn and look east. Rates should increase after 1am as Earth rotates to encounter the dust particles head on. If possible, aim to keep watching through around 3.30am just before the start of astronomical dawn.
As you’re looking for meteors, Mars dominates the view low to the south. After opposition last month, Mars is now slowly dimming. The distance between our two worlds is increasing and through a telescope the apparent size of Mars is now shrinking. The dimming and shrinking are slow at first so there’s still time to get a decent view.
The Milky Way is glorious at this time of year, passing from the north, overhead and down into the south. It represents the light of billions of suns in our own galaxy.