2020-03-01_Australian_Geographic

March. April 23

GEOBUZZ

ILLUSTRATION: SHUTTERSTOCK

LOOKING

UP

Naked eye

Autumn evenings

display the glorious southern

Milky Way. If suburban skies

rob your view of our galactic

glow, its position can easily be

determined. Start with the

pointers and the Southern

Cross. Move westward to the

False Cross then on to the

brilliant stars of Canopus

followed by Sirius.

x1

with Glenn Dawes

x10 0

Small telescope

The constellation of

Musca is best known for two

globular clusters, both within

one degree of bright stars.

These ‘cities of stars’ are quite

different. Large NGC 4372

(near Gamma Muscae) has an

even-surface brightness while

NGC 4833 (near Delta)

distinctly brightens towards

its centre.

x10

Binoculars

Within one field south

of the False Cross in Carina lies

the brilliant open-star cluster

NGC 2516. Within the obvious

round haze are a number of

brighter stars, clearly visible,

that form curved lines, with a

red-coloured star offset from

the centre.

Glenn Dawes is a co-

author of the yearbook

Astronomy 2020 Australia

( Q u a s a r P u b l i s h i n g ).

quasarastronomy.com.au

NEED TO KNOW with Dr Karl Kruszelnicki

DR KARL is a prolifi c broadcaster, author

and Julius Sumner Miller fellow in the School

of Physics at the University of Sydney.

His latest book, Dr Karl’s Random Road Trip

Through Science, comes with augmented reality

features and is published by

HarperCollinsPublishers Australia.

Follow him on Twitter at @DoctorKarl

EASTER’S TIMING

M

OST OF US think of Easter

as something to do with

chocolate Easter eggs.

But aft er a few years of celebrating

Easter we begin to realise that, unlike

Christmas or Anzac Day, it doesn’t

happen on the same date each year.

In fact, Easter can happen anytime

between 22 March and 25 April.

Fortunately, there’s a simple thousand-

year-old formula to work it out.

Back in 725 AD, the Benedictine

monk St Bede, or The Venerable Bede,

defi ned Easter Sunday as: “The Sunday

following the full Moon which falls on,

or aft er, the equinox...”

Let’s work backwards – equinox,

full Moon, Sunday – and start with the

equinox. Its Latin roots are equi meaning

“equal”, and nox meaning “night”.

And it describes when there are equal

hours of daylight and night-time.

(They’re not exactly equal but I’ll

explain that another time.)

Astronomers, however, defi ne the

equinox as being when the line be-

tween day and night crosses both the

North and South poles simultaneously.

There are two equinoxes each year –

in late March and late September.

To work out Easter, fi rst we wait for

the March equinox. Then we wait for

the next full Moon, which can take any-

thing between one and 28 days. Finally,

we wait for the next Sunday, which can

be any time from one to seven days

away. And that’s Easter Sunday.

But that simple defi nition can get

messy when religion takes precedence

over science, as it did last year,

for example.

In the March equinox of 2019,

the line between day and night

crossed the North and South poles

simultaneously at 9.58pm Universal

Time on Wednesday 20 March.

The full Moon occurred about four

hours later, at 1.43am Universal Time,

Thursday 21 March. The very next

Sunday 24 March, should have been

Easter Sunday. But it wasn’t. Easter

Sunday in 2019 was 21 April!

Huh?

Astronomers have determined that

the equinox can happen on 19, 20 or

21 March. But religious leaders says it

occurs only on 21 March.

The next full Moon aft er 21 March

was 19 April. The next Sunday was

21 April – Easter Sunday.

No wonder the supermarkets

hedge their bets by stocking chocolate

bunnies from February.

(23 September)

(21 March)