Page 23 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

NEED TO KNOW

JOURNEY ROUND THE CHANGING NIGHT SKY

THE SUN To understand why the stars, and it’s 3 minutes 56
constellations shift around the seconds shorter than the solar
On its way round the Sun, the Earth spins sky, we fi rst need to consider day. This difference is due
on a tilted axis. Either the northern or the length of a day. Not the to the fact that the Earth, as
southern hemisphere gets more direct regular day of 24 hours, well as spinning on its axis,
sunlight, causing the seasons however, which is called the also orbits the Sun. This time
solar day and is the time it difference between the solar
takes the Earth to spin once and sidereal days, although
on its axis in relation to the short, causes the stars to rise
Sun. No, there’s another day, almost four minutes earlier
called the sidereal day. each day and is why the
This is based on the Earth’s constellations change in the
rotation with respect to the sky through the year.
15 DECEMBER, 7PM
PERIHELION
Earth 147.1 million km
from the Sun

NORTHERN
WINTER SOLSTICE
The shortest day ORION

15 JANUARY, 7PM

DAY AND NIGHT
Earth spins on its axis
once every 23.93 hours
A YEAR
Earth orbits the
Sun in 365.26 days

15 MARCH, 7PM
the northern hemisphere’s winter, Earth is as close to the Sun as it
can get: perihelion happens around 3 January.
The seasons are in fact due to Earth spinning on a tilted axis as it
moves around the Sun, which varies the intensity of sunlight hitting
each hemisphere throughout the year. Model globes of Earth show
this: they lean by 23.5° from the vertical. You can see this lean in
relation to our orbital path around the Sun in the diagram above.
At times, the North Pole tilts 23.5° towards the Sun, while the South
Pole points away by the same amount. For the northern hemisphere,
the day this happens is the longest day (the summer solstice) around
20 June and for the southern hemisphere it’s the shortest day (the
winter solstice). Six months later, the tilt is reversed so that the South 15 MAY, 7PM
Pole points towards the Sun and the North Pole leans away into space.
This marks the shortest day in the northern hemisphere and the
longest day in the southern hemisphere (around 21 December).
As it goes round the Sun, Earth’s axis always tilts in the same
direction in relation to the stars. In the northern hemisphere you can
see this by the fact that the star Polaris is always in the same place
overhead due north.
The spin of Earth and its motion around the Sun doesn’t just
create the seasons. It also explains why our view of the constellations
changes over the year.
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