Transcript Celestial Astro II

Annual Motion
ecliptic
the apparent path of the Sun through the sky
equinox
where the ecliptic intersects the celestial equator
solstice
where the ecliptic is farthest from the celestial equator
zodiac
the constellations which lie along the ecliptic
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Annual Motion
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As the Earth orbits the Sun, the Sun appears to
move eastward with respect to the stars.
The Sun circles the celestial sphere once every
year.
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Annual Motion
The Earth’s axis is tilted 23.5° from being
perpendicular to the ecliptic plane.
 Therefore, the celestial equator is tilted
23.5° to the ecliptic.
 As seen from Earth, the Sun spends 6
months north of the celestial equator and
6 months south of the celestial equator.
 Seasons are caused by the Earth’s axis
tilt, not the distance from the Earth to the
Sun!
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The Cause of the Seasons
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Axis tilt causes uneven heating by
sunlight throughout the year.
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Seasonal Change in Sun’s
Altitude
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The “Figure 8” shows Sun at same time each
day over a year.
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Seasonal changes are more
extreme at high latitudes
Path of the Sun on the summer solstice at the
Arctic Circle
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When is summer?
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Although the solstice
which occurs around
June 21 is considered the
first day of summer.
It takes time for the more
direct sunlight to heat up
the land and water.
Therefore, July & August
are typically hotter than
June.
Image:
http://www.beachchamber.com/newwebsite/photos/
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Why doesn’t distance matter?
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Small variation for Earth — about 3% (but distance
does matter for some other planets, notably Mars and
Pluto).
Surprisingly, seasons are more extreme in N.
hemisphere, even thought Earth is closer to Sun in S.
hemisphere summer (and farther in S. hemisphere
winter) — because of land/ocean distribution
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Precession of the Equinoxes
• The Earth’s axis precesses (wobbles) like a
top, once about every 26,000 years.
• Precession changes the positions in the sky of
the celestial poles and the equinoxes.
 Polaris won't always be the north star.
 The spring equinox, seen by ancient Greeks in
Aries, moves westward and is now in Pisces!
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Sun sign vs. Tropical Sign
Tilt of Earth’s axis towards or away from
sun determines Season
 Calendar is designed to keep same
season in the same month…it accounts for
precession
 Precession changes the place on the
Earth’s orbit where a season and month
occurs
 It is impossible to keep Season, month,
and position in orbit the same…so:
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Horoscope is about one month off!
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Consider extreme case: in 13,000 years, sun’s axis will
rotate from towards the sun to away from sun.
In July, Sun is in conjunction with Leo
In 13,000 years, the sun will be in the same place in its
orbit, and in conjunction with Leo, but the calendar will
say that it is January.
The Zodiac/Horoscope dates were established by
Ptolemy, about 100 AD. Since then the Axis has
wobbled 2/27 of a complete circle—about a months
worth if you are counting.
Astrologers differentiate between the Tropical sign (by
date) and the Sun sign (by position)…good luck!
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The Local Sky
zenith
the point directly above you
horizon
all points 90° from the zenith
Altitude
the angle above the horizon,
Azimuth—angle from North horizon
meridian
due north horizon zenith due south horizon
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To pinpoint a spot in the local sky:
Specify altitude –the angle above
the ground and
-azimuth is the number of degrees east
of North along the horizon.
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Coordinates on the Earth
Latitude: position north or south of
equator
 Longitude: position east or west of
prime meridian (runs through
Greenwich, England)
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Measuring the Sky
We measure the sky in angles, not distances.
 Full
circle = 360º
 1º = 60 arcmin = 60’
 1 arcmin = 60 arcsec = 60”
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Measuring Angles in the Sky
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Venus, Jupiter, Crescent Moon
What is the
The angle
between
Venus and
Jupiter?
5
degrees!
http://www.shoestringastronomy.org.uk/photo/conj/moon_venus_jupiter.jpg
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Phases of the Moon
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Lunar Motion
Phases of the Moon’s 29.5 day cycle
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•
•
•
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•
•
new
crescent
first quarter
gibbous
full
gibbous
last quarter
crescent
waxing
waning
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Why do we see the same face?
Rotation period = orbital period
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Earth and Moon from space
This won’t play on the web, but see PlanetTales!
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Eclipses
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The Earth & Moon cast
shadows.
When either passes
through the other’s
shadow, we have an
eclipse.
Why don’t we have an
eclipse every full & new
Moon?
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Moon’s orbit tilted 5° to ecliptic plane
Crosses ecliptic plane only at the two nodes
 Eclipse possible only when full/new occur near
nodes
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Solar Eclipse
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Lunar Eclipse
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