BBC Sky at Night - UK (2021-08)

DON’T MISS

Everything’s falling into place for a

potentially spectacular display of

Perseid meteors this month. Generally,

low-rate activity occurs between 17 July

and 24 August with the exception of the

peak period 8-16 August, a sharp peak

typically seen around 11–13 August. The

peak timing varies year on year. In 2021 it’s

predicted for 20:00-23:00 BST (19:00-

22:00 UT) on 12 August. Rates naturally

increase after local (non-daylight saving

time) midnight, so the period from

01:00-03:30 BST on the 13th is optimal.

The Perseid shower is a solid performer,

reaching a peak zenithal hourly rate (ZHR)

of 100-plus meteors per hour. This is what

you’d see under perfect viewing conditions

with the meteor radiant overhead and the

ability to see the whole sky. However, the

actual visual hourly rate is often lower.

Two variable factors affect shower

visibility; the weather and the Moon.

There’s little you can do about the

weather except move location if things

look really bad. The Moon either interferes

or it doesn’t. On 12 August, the 4.5-day

old, 20%-lit waxing crescent Moon sets at

22:35 BST (21:35 UT) and won’t interfere.

Meteor showers occur when Earth

passes through dust distributed around a

comet’s orbit. Entering our atmosphere on

parallel paths, perspective causes meteor

trails to appear to emanate from the

same sky location – the shower radiant.

Over the activity period, the radiant’s

position drifts against the background

stars. Peak activity represents us passing

through the densest part of the stream.

A garden lounger is a great viewing

platform. Look for meteors at an altitude

around 60Ō in any direction. While longest

trails are seen 40–140Ō from the radiant,

towards the radiant expect short trails. A

look in the opposite direction to the radiant

will reveal trails that appear short and

converge to a point called the anti-radiant.

Perseid displays often exhibit bright

events, many of which show what

appears to be an after image of the trail,

which is a weakly glowing column of

BEST TIME TO SEE: 8–16 August,

peak night 12/13 August

46 BBC Sky at Night Magazine August 2021

THE BIG THREE The three top sights to observe or image this month

AL

L^ P

ICT

UR

ES
:^ P

ET
E^ L

AW

RE

NC

E

SPECTACULAR

PERSEIDS

ionised gas. This ‘meteor train’ fades from

view as the energy in the ionised atoms is

given up. High altitude winds may also

affect the train, distorting its shape.

> See the ‘Perseid Perfection’ feature on

page 28 for more on this year’s shower

M31

CASSIOPEIA

ANDROMEDA

CAMELOPARDALIS

URSA CEPHEUS

MINOR PEGASUS

PERSEUS

TRIANGULUM

ARIES

AURIGA

a

_

a

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a _

a

_

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_

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` _

_

`

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a

d

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30 Jul

10 Aug

20 Aug

Peak activity

11/12/13 August

15 Aug

25 Aug

5 Aug

25 Jul

20 Jul

15 Jul

Polaris

Algol

Mirfak

Hamal

Alpheratz

Capella

DoubleCluster

Perseid

radiant

Great Square

of Pegasus

W

House

11 Aug

12%-lit waxing crescent

sets at 22:20 BST (21:20 UT)

Moon phase

(peak nights)

12 Aug

20%-lit waxing crescent

sets at 22:35 BST (21:35 UT)

View the best of

the Perseids on the

night of the 12/13

August between

23:15–03:20 BST

(22:15–02:20 UT)

Make sure your eyes are properly dark adapted

from around 22:30 UT (21:30 BST) on 12 August