PHY134 Introductory Astronomy The Sun and the Moon 1
Slide 2
So Far Stars fixed on a large celestial sphere that rotates
daily East to West about an axis through Earths poles Declination
is celestial longitude. Observers zenith is at Declination =
Latitude Right Ascension is celestial longitude. Observers zenith
is at RA = Sidereal Time Sidereal time changes by about 1h/h Sun
moves to East relative to stars once a year 2
Slide 3
Sky Charts N WE S Zenith Fixed Azimuth Horizon Fixed Altitude
Pole Fixed RA Equator Fixed Decl.
Slide 4
Clocks Time from noon to noon is a bit (1/365 of a day or about
4min) longer than time it takes Earth to turn 360 A (mean) solar
day is longer than a sidereal day Our clocks (LT) keep solar time
so run slower than sidereal clock (ST) 24 sidereal hours = 23h 56m
4s 4
Slide 5
Finding Sidereal Time By convention ST LT on September 21 D
days later (earlier) ST LT +/- D4m This is approximate. In any
event ignores time zones and Daylight Savings Time On
December/March/June 21 ST LT + 6/12/18 h 5
Slide 6
It Tilts Earths axis is tilted 23.5 from perpendicular to orbit
Celestial equator tilted 23.5 from plane of orbit ecliptic Suns
orbit along Celestial sphere ecliptic - tilted 23.5 from Celestial
equator Suns Declination changes between 23.5 and -23.5 Ecliptic
meets equator at Vernal/Autumnal equinox at 0h/12h RA 6
Slide 7
Seasons When Sun North/South of equator Days longer in
North/South Sun higher in the sky in North/South Climate warming in
North/South cooling in South/North Inside Arctic circle Sun becomes
circumpolar/never rises (reverse for Antarctic circle) At equinox
day/night equal everywhere Between tropics Sun is at Zenith once a
year 7
Slide 8
How High is Sun at Noon? We are at Latitude 36N At equinox At
summer solstice At winter solstice 8
Slide 9
Why Mean? 24h is an average Solar day Suns RA increases over
the year but not uniformly Sun moves around ecliptic almost
uniformly but ecliptic is tilted near equinoxes and parallel to
equator near solstices. So Eastward motion fastest near solstices.
Almost Earth very slightly nearer Sun in January 9
Slide 10
It Also Wobbles The Earths axis wobbles like a spinning top
precession Celestial axis wobbles. North pole moves to the West in
a circle of radius 23.5 every 26,000 years relative to stars So
does celestial equator hence precession of the equinoxes
Coordinates of stars change too epoch J2000 Age of Pisces gives way
to age of Aquarius ca. 2600 10
Slide 11
Moon Moves Too Like Sun, Moon moves around celestial sphere as
it orbits Earth West to East Moon is faster: orbits in a sidereal
month (27.32 days) RA increases by 48min per day Spin locked to
orbit same side always faces Earth Moon moves relative to Sun by
44min per day Full rotation relative to Sun in synodic month (29.53
days) Position relative to Sun controls rise/set times as well as
phases 11
Slide 12
Moons Declination Moons orbit inclined 5 to ecliptic about line
of nodes Like Sun, Moon higher in Summer Twice a year line of nodes
aligns with Sun: Eclipse Season Tilt precesses to the West every
18.6 years so twice an eclipse year of 346.6 days At New/Full Moon
during eclipse season have Solar/Lunar eclipse 12
Slide 13
Solar Eclipse Moon almost same angular size as Sun With near
perfect alignment can completely obscure Sun from up to 250km
shadow total eclipse More common partial eclipse When Moon farthest
from Earth annular eclipse 13
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Lunar Eclipse Moon enters Earth shadow from West Eclipse can be
total or partial. Penumbral eclipse when Moon in partial shadow
dims slightly During totality Moon illuminated through atmosphere
looks red 16
Slide 17
Fun with the Moon Moon appears larger near horizon This is a
psychological illusion not shared by optical instruments Various
theories as to mechanism Can see dark part of crescent Moon old
moon in new moons arms This is physical viewing dark part by
reflected Earthlight 17
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Slide 19
Signs of the Times Astronomy and timekeeping are always closely
related we want our time to match what happens. Our 24-hour days
are adjusted to mean solar day. Our months are approximately lunar.
Our years match orbit 365.2564 days is a sidereal orbit. Tropical
orbit is 365.2422 days (precession). Julius Caesar got 365.25 so
invented leap years. Pope Gregory XIII (1582) corrected for
the.0078 19
Slide 20
Summary Our cosmos now has moving parts Sun moves around
Celestial Sphere to the East, completes one revolution in a year.
The ecliptic tilted relative to celestial equator by 23.5 about
equinoxes and precesses West every 26,000 years Moon moves around
Celestial Sphere to the East, completes one revolution in a month.
Moons orbit tilted relative to ecliptic by 5 about line of nodes
and precesses West every 18.6 years The model now explains
day/night, lunar phases, eclipses What else moves? 20