Download Celestial Sphere, Celestial equator, N

Celestial Sphere, Celestial equator, N. Celestial pole, S. Celestial pole
One way to think of the stars in the sky is to imagine that they are projected onto a giant
celestial sphere centered on the observer. At any one time, we can only see half this
celestial sphere. Because of this, the stars and sun appear to trace circles on the celestial
sphere – they appear to rise in the east and set in the west because of the rotation of the
Earth about its axis.
The stars and location of the Sun we see depends on the time of the year and on our
position on Earth.
The Sun appears to circle the Celestial sphere once a year on a path called the ecliptic.
Ancient civilizations imagined stars grouped together according to legends and folklore –
such groups are called constellations. Those constellations lying near the ecliptic and
hence those constellations which the Sun appears to go through during the course of a
year are called the zodiac. Another complication to what stars we can see at a certain time
of the year and at a certain location is the fact that the Earth’s rotation axis is tilted at 23.5
degrees to the plane of the ecliptic.
The zenith is the point directly overhead. Thus different people, in different locations on
earth have different zeniths. The boundary between earth and sky is the horizon. Your
meridian is an imaginary half circle drawn on the sky starting from the horizon due north
and going through your zenith and ending at the horizon due south.
On the Earth, we measure position by latitude and longitude: latitude measures N/S with
zero degrees being on the equator and 90 degrees being on either the North or South pole.
Longitude measures E/W with zero degrees being called the prime meridian – this goes
through the North and South poles and Greenwhich UK. On the sky, in order to describe
the positions of objects on the celestial sphere, the equivalent of latitude and longitude
are declination and right ascension (RA).
The earth’s daily rotation makes the stars appear to rotate around us. Because we only see
half the celestial sphere at any one place, this simple rotation of the stars looks more
complicated and actually makes it appear that stars rise and set. The set of stars that rise
and set varies with location on Earth and time of year. Circumpolar stars are stars which
are relatively high in the sky and never seem to set but instead they circle the north/south
celestial pole. It is just luck that the pole star, or Polaris, is close to the north celestial
pole. Thus the Northern and Southern hemisphere have different sets of circumpolar
stars.
At any location, the altitude of the north celestial pole is equal to your latitude.
The fact that the Earth’s axis of rotation is tilted to the plane of the ecliptic is responsible
for the seasons, NOT the distance the Earth is from the Sun. The Earth maintains this
direction in space throughout its rotation about the sun. Longer days in the summer occur
because the Sun spends a longer time in the sky, above the horizon – again because of the
tilt of the Earth’s axis to the ecliptic. In the Northern hemisphere, when the tilt of the
Earth’s axis is toward the Sun, the Sun’s rays strike the ground at a more direct angle and
thus heat up the ground more. During winter in the northern hemisphere, the tilt of the
Earth’s axis means the Sun’s rays strike the ground in a more slanted fashion – the energy
is more spread out and thus heats up the ground less. This difference in the angle at which
the Suns rays strike the ground between Winter and Summer is more pronounced the
closer you get to the poles. Thus, equatorial regions have less well defined seasons.
Similar arguments apply in the Southern hemisphere.
Know definitions of sidereal day, solar day, synodic month, sidereal year, tropical year
winter, summer solstice and fall and autumnal equinox. Know why the solar day is longer
than the sidereal day. Know the definitions of the tropic of Cancer and tropic of
Capricorn.
Know what is precession and know that Earth’s axis of rotation is precessing about an
axis – currently the Earth’s rotation axis points to the Polaris the north star but it will
point to Vega in about 15,000 years. The period of this precession is about 26000 years.
The Copernican revolution – who were the main people involved in this and what was
their contribution.
Gravity, mass and weight on different planets. Newton’s laws of motion. The
electromagnetic spectrum, Light as a wave and as a photon. Conservation of energy,
momentum, kinetic and potential energy.