Download 3.2dl Apparent motion of stars

3.2d
describe what is meant by the term ‘circumpolar stars’ and explain the
connection between the apparent motion of stars and the Earth’s rotation
3.2l
explain the apparent east-west motion of the night sky
Stars appear to move across the night sky. This is apparently happening – but
actually the stars only ‘move’ because of the rotation of the Earth. As the Earth
rotates in an anticlockwise direction, the east-west motion of the night sky becomes
apparent. The position of the distant stars is fixed, we are the ones moving.
POLARIS
Stars inside the
circle never sink
below the
horizon
CIRCUMPOLAR
STARS
Stars, like the constellation LEO,
outside the circle, sink below
the western horizon
STARS ARE NOT
CIRCUMPOLAR
The rotation of the Earth on its axis in an anticlockwise direction is the reason the
stars track across the sky. As the axis passes close to Polaris, the Pole Star, this
appears to stay in one place and the other stars move around the Pole Star.
During the night, a constellation like Leo will rise in the east and move steadily
towards the west. Eventually, as it is not circumpolar, it will sink below the horizon.
Other stars of the Zodiac track in similar fashion and whilst Leo disappears from
view, Virgo may still be seen.
We know that the Earth moves in orbit around the Sun. If the Sun is stationary, the
Earth must be spinning or we would not have day and night. One side of the Earth
would be permanently day and the other side would be night.
EAST
A
DAWN
Viewed from above, as a
person at point A is at dawn
on the Earth, the Sun rises
in the East - or on their left
if they were facing South.
SOUTH
A
Viewed from above, as a person
at point A is at midday on the
Earth, the Sun rises to its
highest point - or straight in
front if they were facing South.
MIDDAY
WEST
A
DUSK
Viewed from above, as a
person at point A is at dusk
on the Earth, the Sun sets in
the West - or on their right
if they were facing South.
SUN
EAST
EAST
WEST
The apparent motion of the Sun (and Moon)
as we see the objects move through the sky
- the Earth must be rotating anticlockwise
for the Sun to rise in the east and set in the west
Picture credits : (Earth) NSSDC/GSFC/NASA - part of a larger image of the Earth and Moon
taken by the Galileo space probe (Sun) SOHO/ESA&NASA