Ch 2 The Sky http://www.blinn.edu/brazos/natscience/kbyboth/phys1411-A3cis.html Objectives Use of the celestial sphere and angular measurement Constellation and Star names Introduce the magnitude scale Relative motion of common naked eye objects 2-1 The Stars Constellations vs Asterisms
Constellations vs Asterisms
Two ways to name stars
The brightness of stars, apparent visual magnitudes
The “Obvious” View
On a clear dark night we can see stars to 6th magnitude with the unaided eye. Magnitude scales will be explained later.
The human mind by its nature combines these into patterns. This tendency is useful for organization but can lead to misconceptions about the true positions of the stars.
Our ancestors named these groupings in support of their belief systems and oral traditions. Many of the ones we know today originated in Mesopotamina over 5000 years ago.
People connected the stars into patterns we call constellations. Names and myths for constellations are culture dependent, but interestingly enough the stars grouped together are frequently similar. Interpretations are different.
Al Jabbar the Giant (ancient Syrians)
White Tiger to the Chinese
Prajapati..a stag in ancient India.
Orion is prominent in winter evening sky.
While we scoff at the field of astrology now it is not much of a stretch to see why the ancients thought a cause and effect relation existed between the location of heavenly objects and events in their everyday lives. Modern astronomy owes much to the practice of astrology. Many early astronomers and mathematicians made a living casting horoscopes for wealthy clients.
There are 88 constellations . Regions of the sky are now named by the most prominent constellation in the area.
Constellations Near Orion
Big Dipper a subset of Ursa Major is an asterism, a smaller less well defined grouping of stars.
The great square of Pegasus is another asterism.
The brightest stars were named thousands of years ago.
Most of our constellation names come from Greek versions translated into the Latin, but most star names come from ancient Arabic.
Sirius .. The Scorched One,
Capella .. Little She Goat
Alderbaran… Follower of the Pleiades
Another more useful method uses the name of the constellation and a Greek letter prefix to indicate the relative brightness.
From brightest to dimmest.
Hipparchus Gk. 160-127 BC
Originally 6 classes.
The human eye has been replaced by instruments and a scale has been exactly defined.
Vega ~ 0 mag Antares(alpha Scorpio) 0.92
Negative I.e. very bright objects are possible
Full Moon –12.5
SAO 132071, 3.35
Saiph , 2.06
The Celestial Sphere The sky at any moment
Celestial Sphere Reference Points
The Sky and Compass points,
The motion of the sky
Precession, north moves around…slowly
Zenith-directly over head
Nadir- 1800 away from zenith
North Celestial Pole-directly above the Earth’s north rotational axis
South Celestial Pole-above south pole
The great circle passing directly overhead and through the celestial poles is your meridian.
The zero reference meridian passes through the observatory at Greenwich, England.
900 right angle
e.g.Horizon to Zenith, due north to due east
1800 straight angle: Zenith to nadir, due east to due west
Width of little finger… ~10
Caution!!Be careful where youuse #4 .
Stand on the north pole and the celestial north pole is at your zenith.
Walk 300 south and the pole moves 300 north. You are now at 600 N Latitude.
Move to 300 N Latitude and the North Celestial Pole is 300 up from your northern horizon and 600 north of your zenith.
Stars move across the sky from east to west over the course of a night. This is of course due to the rotation of the Earth on its axis towards the east.
Which stars are directly overhead depends on our location on the Earth.
The relative positions of the stars remain the same. People thought the stars must be attached to a Celestial Sphere surrounding the Earth.
The stars above your northern horizon that never set depend on your latitude.
Polaris, North Celestial Pole, is the same number of degrees above the northern horizon as your latitude.
The patterns of stars have been stable over time scales large compared to the lifetimes of men and even their cultures. Stars were thought to live forever.
This stability led to the use of stars as navigational guides, and the repeating nature of the times of which stars are visible led to their value as calendars.
Knowing when to plant crops or to break winter camp to meet migrating game is a real survival tool
Long term changes.. Over centuries. Precession
Daily.. Diurnal motion.
A top precesses in a conical motion due to the effect of the earth’s gravity.
The Earth precesses due to the combined effect of the Sun and Moon on the bulge at the Earth’s equator.
The precession has a 26.000 year cycle.
4800 years ago the pole was close to Thuban. The pole will be closest to Polaris in ~2100 .
A spinning object affected by an outside force will wobble on its axis. This is called precession. This causes the axis of rotation to trace out a circular path. The Earth’s cycle of precession lasts about 26,000 years.
Due to the precession a sidereal year is about 20 minutes longer than a tropical year, 365.256 mean solar days. If we did not use the tropical year the seasonal months would change.
Time laps photograph
We now know this motion is due to the rotation of the Earth on its axis.
We still use the concept of the Celestial Sphere to learn our way around the sky.
We use angular separations and positions on the sky to discuss the locations of stars.
Earth’s Orbital Motion
The 24 hour Solar Day , from noon to noon, is the basic social time unit. By definition 1 solar day is 24 hours.
The daily progress of the Sun and other stars across the sky is called diurnal motion.
Each night the celestial sphere shifts a little across the sky.
… a day measured by the star positions is called a sidereal day.
Sidus ,latin for star.
The Earth rotates on its axis and revolves around the Sun.
The solar day is 3.9 minutes longer than the sidereal day.A sidereal day is 23h56m long.