PHOTO CREDIT, CLOCKWISE FROM TOP: SHUTTERSTOCK; WIKIMEDIA; JIM BENSON / WIKIMEDIA; OPPOSITE: STEVE WILSONTHE TOWN OF Karumba – at the Gulf of
Carpentaria’s bottom right-hand corner
- experiences only one high tide and one
low tide each day. What’s going on?
The gravitational forces of our
moving Moon and the Sun attract
Earth’s oceans and suck them into
corresponding watery bulges. The
normal rotation of our planet
brings any point on Earth’s surface
towards, under, and then away
from these bulging walls of water.
This gives the impression of tides - the ocean rising and falling.
But the situation is actually
more complicated. The Moon
doesn’t orbit directly above the
Equator, but instead swings above
and below it. The oceans have
different depths, continents have
odd shapes and get in the way of
the bulging water. The Earth is
tilted some 23° from the vertical; the
Earth spins every 24 hours while the
Moon takes a month to loop around our
Earth; water is slowed by friction as it
moves across the ocean
floor – and there are a lot
more confounding issues.
When you factor all this
in to the equations, you get
about 120 different possible
tides each day. There are tides that
happen once a day, twice a day, three
times a day, four times a day, and so
on. But the twice-a-day tides, which
have the biggest energy and height,
are most common.
In the Gulf of Carpentaria, most
of the tidal energy comes from the
Indian Ocean. Hardly any gets through
the little 150km gap between Cape
York and New Guinea. So the Gulf is
virtually a closed body of water.It takes 12 hours for a water wave
to slosh across the Gulf of Carpentaria
from east to west – and another 12
hours to bounce off and come back.
This 12-hour period is (coincidentally)
the same as the time between two
high tides (or two low tides). In the
Gulf, the incoming tides ‘cancel out’
the outgoing tides, so there are no
twice-per-day tides.
Now, the next most energetic tides
are the once-a-day tides. And that’s
why you have only one tide a day at
Karumba. A single tide each day
happens for exactly the same reason
in the Gulf of Thailand, the Persian
Gulf and the Gulf of Mexico.w ith Dr Karl Kruszelnicki
Just one tide a day
NEED
TO KNOWDR 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, The Doctor,
is published by Pan Macmillan. You can follow
him on Twitter: @DoctorKarl.AFTER 19 DAYS of flying,
26-year-old British aviatrix
Amy Johnson touched
down in Darwin to world
acclaim – the first woman
to fly solo from England
to Australia. Born in Hull,
England, on 1 July 1903,
Amy’s interest in aeroplanes
began just two years before her
record-breaking flight to Australia.
And it was only in 1929, a year before
this epic journey of almost 16,000km, that she
was awarded her flying licence, after 85 hours
of flying training – mostly through the London
Aeroplane Club. Before that, Amy hadn’t even
flown over the English Channel. And yet,
convinced that women could be as proficient
at flying as men, she set off on her own from
London in her DH Gipsy Moth on 5 May
1930 to fly halfway around the world and into
the record books.On this day
A number of explanations have been put forward
to explain the ‘dawn chorus’, when birds are at
their most vocal. The dawn chorus can start at
different times, usually 30–90 minutes before
sunrise, depending on the species of bird and
season. The intensity of the chorus is loudest
during the breeding season. Because it is
generally males that do most of the singing and
calling, the most likely explanation is that they
are reconfirming their territories and letting
females know their whereabouts. As light levels
are poor this early in the day, foraging is not
practical, and so males are taking the
opportunity to warn off rival males
while females are listening out
for a suitable mate based
on song quality.Ask an expert
Why do birds sing at sunrise?Professor Les Christidis, one of the world’s leading experts
on Australian birds, Southern Cross UniversityQ
A