Chapter 2: The Sky - Siena Collegerfinn/courses/astro/chapter2.pdf1 Chapter 2: The Sky Today,...
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1 Chapter 2: The Sky Today, constellations are well-defined regions of the sky, irrespective of the presence or absence of bright stars in those regions. Constellations In ancient times, constellations only referred to the brightest stars that appeared to form groups, representing mythological figures. The Celestial Sphere • Zenith = Point on the celestial sphere directly overhead • Nadir = Point on the c.s. directly underneath (not visible!) • Celestial equator = projection of Earth’s equator onto the c.s. • North celestial pole = projection of Earth’s north pole onto the c.s.
Transcript of Chapter 2: The Sky - Siena Collegerfinn/courses/astro/chapter2.pdf1 Chapter 2: The Sky Today,...
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Chapter 2:
The Sky
Today, constellations are well-defined regions of thesky, irrespective of the presence or absence of bright
stars in those regions.
Constellations
In ancient times,constellations only
referred to the brighteststars that appeared to
form groups,representing mythological
figures.
The Celestial Sphere• Zenith = Point on
the celestial spheredirectly overhead
• Nadir = Point onthe c.s. directlyunderneath (notvisible!)
• Celestial equator =projection ofEarth’s equatoronto the c.s.
• North celestial pole= projection ofEarth’s north poleonto the c.s.
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Stand and spin...• Orientation
– North = your head– East = left hand– West = right hand– South = feet
• Spin counter-clockwise as seen from above.• In which direction do you see objects first
(or rise)?• In which direction do you see objects last
(or set)?
The Sky
• We can use The Sky software to illustratethe apparent motion of the sun and stars...
Apparent Motion of theCelestial Sphere
Apparent Motion of the Celestial Sphere II
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The stars of aconstellation only
appear to beclose to one
another.
Usually, this isonly a projection
effect.
The stars of aconstellation maybe located at verydifferent distances
from us.
Stars are named by a Greek letter (α, β, γ,) according totheir relative brightness within a given constellation + the
possessive form of the name of the constellation:
Betelgeuse = α Orionis,
Rigel = β Orionis
Betelgeuse
Rigel
Orion
The Celestial Sphere The Celestial Sphere
• Positions of starsindicate location onearth - latitude
• Elevation of Northstar = geographiclatitude (northernhemisphere)
90o -
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The Magnitude ScaleFirst introduced by Hipparchus(160 - 127 B.C.):
• Brightest stars: ~1st magnitude• Faintest stars (unaided eye): 6th magnitude
More quantitative:
• 1 mag. difference gives a factor of 2.512 in apparentbrightness (larger magnitude = fainter object!)
• 1st mag. stars appear 100 times brighter than 6th mag.stars
The magnitude scale system can be extendedtowards negative numbers (very bright) and
numbers > 6 (faint objects):
Sirius (brightest star in the sky): mv = -1.42
Full moon: mv = -12.5
Sun: mv = -26.5
Precession (I)
Gravity is pulling on a slanted top. => Wobbling around the vertical.
The sun’s gravity is doing the same to Earth.
The resulting “wobbling” of Earth’s axis of rotation around the vertical w.r.t.the ecliptic takes about 26,000 years and is called precession.
Precession (II)As a result of precession,
the north celestial polefollows a circular patternon the sky, once every
26,000 years.
It will be closest to Polaris~ A.D. 2100.
~ 12,000 years from now,it will be close to Vega in
the constellation Lyra.
There is nothing peculiar about Polaris at all(neither particularly bright nor nearby etc.)
