Download Planetarium Key Points

Pepperdine University - Planetarium Key Points for the lesson of 2/27/2007
Piero Ranfagni [email protected]
1. Celestial sphere
 The stars seem numberless and there are actually more than 2 billions of stars in
the system we live in (Milky Way), but only 3000 stars are visible at naked eye
 What we see is NOT what it is actually, the response of our eye is logarithmic not
linear
 All celestial objects seem at the same distance from us, hence we think to be at
the center of an huge sphere
 Constellations exist becouse of celestial sphere and becouse for living beeings is
natural pattern recognition
 Constellation shape change with epoch and observer position; shape is not for
ever becouse of star’s proper motion
 Constellations and asterisms; we use structures invented by assirian priests in
XII century BC: Orion, Ursa Major, Ursa Minor, Scorpion, Gemini, Taurus, the
Summer Triangle.....
 Using a motionless sphere we can define a great circle, the horizon, and its
poles, zenit and nadir
2. The daily motion of the sphere
 From Est to West around an axis that seems fixed on the sphere (for short
periods of time)
 The motion and the sphere define two poles and an equator, we can use some
stars to find them; Polaris for NCP and Southern Cross and Centaurus for SCP,
Orion Belt for the equator
 Latitude is the elevation of the visible pole and, roughly, of Polaris
 The motion of the sphere was the source for time telling, but the time scale that
comes from is NOT uniform: rotation is slowing down, the day is longer and
longer at the rate of 2 ms a century
 From the motion we can distinguish circumpolar, rising and setting, not visible
stars; their definition depends on the observer’s latitude, so the same stars
become visible, circumpolar or invisible at different latitudes
 At our latitude polar axis is oblique in respect of the horizon and the paths of
each object in its daily motion is likewise oblique; at the North Pole the polar axis
is vertical, the celestial equator overlap the horizon and paths are horizontal; at
the geographical equator the polar axis is on the horizon, the equator is passes
through the zenit and the paths are vertical
3. Fixed and wandering stars
 All celestial object are carried from the daily motion of the sphere at the rate of 15
degree an hour, 1 deg every 4 minutes
 Fixed stars seem to be engraved on the surface of celestial sphere
 Mobile stars move along the ecliptic line from West to Est, that is their direct
motion; some of them sometime move in retrograde motion from Est to West
 Also the Sun seems to move along the ecliptic line, its apparent path among the
stars was completely described by Mesopotamian People three thousands of
years ago and Greek People learnt from them. Perhaps eliacal rising of a star
was used
 The Moon moves actually around us at the variable rate of around 14 degree a
day
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The motions of the Sun and the Moon along the ecliptic are the base of
calendars
The Sun rising and setting points, Sun elevation at noon and the lenght of its
daily arc depend on the date and latitude
The annual Sun trip defines seasons, solstices and equinoxes
4. The precession of equinoxes
 Since most of the members of the solar system orbit in the same plane, close to
the ecliptic, they tend to pull the equatorial bullge of the Earth towards it and
most of this “flattening torque” is caused by the Moon and the Sun. But the Earth
is rotating and therefore the torque cannot change the inclination of the equator
relative to ecliptic, istaed the rotation axis turns in a direction perpendicolar to the
axis and to the torque, thus describing a cone once in roughly 26000 years
 Polaris is not the same star and some time there is no star close to Celestial
Poles; Vega is nowaday far away from NCP, but it will be very close in 13000
years
 The equinoxes seem to reach western horizon before stars, from this, the word
precession
 The position of equinoxes and solstices among the stars change continously and
change likewise the stars you observe in the same seasons in different epochs.