Dominic Ford, Editor
From the Meteor Showers feed
The Daytime Sextantid meteor shower will be active from 9 September to 9 October, producing its peak rate of meteors around 27 September.
Over this period, there will be a chance of seeing Daytime Sextantid meteors whenever the shower's radiant point – in the constellation Sextans – is above the horizon, with the number of visible meteors increasing the higher the radiant point is in the sky.
Seen from Cambridge , the shower will not be visible before around 04:25 each night, when its radiant point rises above your eastern horizon. It will then remain active until dawn breaks around 06:09.
The radiant point culminates (is highest in the sky) after dawn – at around 10:00 EDT – and so the shower is likely produce its best displays shortly before dawn, when its radiant point is highest.
At this time, the Earth's rotation turns Cambridge to face optimally towards the direction of the incoming meteors, maximising the number that rain vertically downwards, producing short trails close to the radiant point. At other times, there will be fewer meteors burning up over Cambridge, but those that do will tend to enter the atmosphere at an oblique angle, producing long-lived meteors that may traverse a wide area of the sky before completely burning up.
The shower is expected to reach peak activity at around 13:00 EDT on 27 September 2018, and so the best displays might be seen before dawn on 27 September and after the radiant rises on 27 September.
At its peak, the shower is expected to produce a nominal rate of around 5 meteors per hour (ZHR). However, this zenithal hourly rate is calculated assuming a perfectly dark sky and that the radiant of the shower is situated directly overhead. In practice, any real observing sight will fall short of these ideal conditions. The number of meteors you are likely to see is thus lower than this, and can be estimated using the ZHR formula.
From Cambridge, the radiant of the shower will appear at a peak altitude of 18° above your horizon, and on the basis of this, we estimate that you are only likely to see only around 1 meteors per hour, even at the shower's peak, since the radiant will be relatively low in the sky.
The Moon, in Pisces, will be only 4 days past full phase at the shower's peak, presenting significant interference throughout the night.
The origin of the shower
Meteor showers arise when the Earth passes through streams of debris left behind in the wake of comets and asteroids. Over time, the pieces of grit-like debris in these streams distribute themselves along the length of the parent object's orbit around the solar system.
Shooting stars are seen whenever one of these pieces of debris collides with the Earth's atmosphere, typically burning up at an altitude of around 70 to 100 km.
On certain days of the year the Earth's orbit passes through particularly dense streams, associated with comets or asteroids which have vented particularly large amounts of solid material to space, and this gives rise to an annual meteor shower. Such showers recur on an annual basis, whenever the Earth passes the particular point in its orbit where it crosses the particular stream of material.
The meteors that are associated with any particular meteor shower can be distinguished from others because their paths appear to radiate outwards from a common point on the sky, which points back in the direction from which their orbital motion brought them.
This is because the grit particles in any particular stream are travelling in almost exactly the same direction when they cross the Earth's orbit, owing to having very similar orbits to the parent object they came from. They strike the Earth from almost exactly the same direction, and at the same speed.
To see the most meteors, the best place to look is not directly at the radiant itself, but at any dark patch of sky which is around 30–40° away from it. It is at around this distance from the radiant that the most meteors will be seen.
By determining the position of this radiant point on the sky, it is possible to work out the orbit of the stream giving rise to any particular meteor shower. It is sometimes even be possible to identify the particular body responsible for creating the debris stream, if there is a known comet or asteroid with a very similar orbit.
The radiant of the Daytime Sextantid meteor shower is at around right ascension 10h00m, declination 0°N, as shown by the green circle on the planetarium above.
|The sky on 27 September 2018|
18 days old
All times shown in EDT.
The position of the radiant of this shower, and its predicted hourly rate, were taken from International Meteor Organisation's List of Meteor Showers.