The Ursid meteor shower will be active from 17 December to 26 December, producing its peak rate of meteors around 22 December.

Over this period, there will be a chance of seeing Ursid meteors whenever the shower's radiant point – in the constellation Ursa Minor – is above the horizon, with the number of visible meteors increasing the higher the radiant point is in the sky.

From Cambridge the radiant point is circumpolar, which means it is always above the horizon and the shower will be active throughout the night.

The radiant point culminates (is highest in the sky) after dawn – at around 08:00 EST – 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 04:00 EST on 22 December 2020, and so the best displays might be seen before dawn on 22 December.

Observing prospects

At its peak, the shower is expected to produce a nominal rate of around 10 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 55° above your horizon, and on the basis of this, we estimate that you may be able to see up to 8 meteors per hour at the shower's peak.

The Moon, in Pisces, will be around first quarter phase at the shower's peak, but will set at 00:40 and pose no interference later in 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.

All of the meteors associated with any particular shower appear to radiate from a common point on the sky (not drawn to scale).

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 parent body responsible for creating the Ursid shower has been identified as comet 8P/Tuttle.

The radiant of the Ursid meteor shower is at around right ascension 14h20m, declination 75°N, as shown by the green circle on the planetarium above.

The sky on 22 Dec 2020

Source

The position of the radiant of this shower, and its predicted hourly rate, were taken from International Meteor Organisation's List of Meteor Showers.

Image credit

© Jacek Halicki 2016. Perseid meteor seen in 2016 from Poland.

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