Events in DARK text happen during darkness. LIGHT text during daylight,
i.e. we can't see them.
Note: Times are calculated for Liverpool and will vary slightly across the UK
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Each month, we identify the best constellations to be seen from the UK between 9pm and 10pm.
This month we'll be looking for the spring constellations of Hydra, Leo
and Ursa Major,
which can be found towards the south in the April night sky.
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Hydra - is said to represent a twisting 'Snake' that stretches across the
night sky. It is named after the nine-headed water serpent that appears in Greek mythology.
Hydra is the largest of the 88 modern constellations and covers 3.16% of the entire sky. Despite
its size, it contains only one reasonably bright star, known as Alphard. The
lack of bright stars makes this constellation a challenge to trace out on all but the
darkest of nights. One of its stars, R Hydrae, is a type of star know as a Mira Variable.
This variable star can be seen to pulsate in brightness over the course of 389 days. At
maximum brightness the star can be seen with the naked eye, but just over 6 months later it
is over 1300 times dimmer and can only be seen with a small telescope. The instability is due
to the star coming towards the end of its lifetime.
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Leo - is named after the Latin word for 'Lion', and is one of the few constallations
that resembles the object it is meant to represent. It is also one of the 12 constellations through
which the ecliptic passes, and is therefore one of the signs of the zodiac. The ecliptic is
an imaginary line across the night sky that the planets never stray far from during their orbits
of the Sun. It marks out the plane of our Solar System,
or alternatively, the axis about which it rotates. Leo is the 12th largest constellation and
contains 3 bright stars, the brightest being Regulus, which also happens to be the 22nd
brightest star in the whole night sky. At just 77 light-years
from the Sun, Regulus has about 3.5 times the Sun's mass and is a young star of only a few hundred
million years - just a baby in terms of how long stars can live. It is spinning extremely rapidly,
with a rotation period of only 15.9 hours, compared with around 30 days for the Sun, and this causes
it to bulge out in the middle. If it were rotating only 16% faster, the centripetal force would
overcome gravity and the star would tear itself apart.
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Ursa Major - are the Latin words for 'Great Bear', and is a constellation that
is visible throughout the year from the UK. It is the 3rd largest constellation and contains
6 bright stars. Ursa Major is also one of the most well known constellations, because part of it
(circled on the diagram) forms the easily recognisable shape of an old plough or saucepan. Indeed
the group of stars in question is well known throughout the UK as the Plough. The Plough
is important for us to know about because it provides a great way to find the North Star (or Pole Star),
officially known as Polaris. By using the two 'pointer' stars' on the right-hand edge of
the Plough (centre of Ursa Major), we can trace a straight line up to Polaris. Because Polaris
is sat directly above the rotation axis of the Earth, it doesn't move during the night, and if
you were to look at it for any length of time you would see other stars circling around it. This
means that on a clear night we can always find out where North is by looking for this famous star.
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