Jun 23, 2015
• 88 constellations in the sky. Those in the northern celestial hemisphere named by the Greeks. Constellations in the extreme southern sky were named in modern times.
• Subsets of constellations are called asterisms. For example, the “Big Dipper” is an asterism of the constellation Ursa Major (the big bear).
• While constellations were named by the Greeks, Arabic names have been mostly adopted for individual stars. Stars within a constellation are assigned Greek letters in accordance with their brightest. For example, the brightest star in Orion has the Arabic name Betelgeuse, but it is also known as Orionis. Orion’s second brightest star, Rigel, is also called Orionis.
• The twelve constellations lying along the projection of the Earth’s orbit onto the sky (the ecliptic) comprise the zodiac.
The Constellations
Winter Sky – Facing North
Winter Sky – Facing South
Orion as Depicted by
J. Hevelius(1690)
(Note that this is as viewed from outside
celestial sphere)
Spring Sky – Facing North
Spring Sky – Facing South
Celestial Motions
• The rotation of the Earth about its spin axis once every 24 hours causes diurnal effects including day and night and the rising and setting of celestial objects.
• The revolution of the Earth about the sun once every 365.2422… days produces annual effects such as the sun appearing to move with respect to the stars along a path in the sky called the ecliptic. The twelve constellations lying along the ecliptic comprise the zodiac.
• The apparent motions of celestial objects on the sky are the combined result of diurnal and annual motions and, in the case of the planets their own orbital motions around the sun.
The Celestial Sphere
celestial equator
celestialsphere
south celestial pole
north celestial pole
south pole
north pole
equator
Earth’sspinaxis
N
S
Equator
* Greenwich
* Atlanta
longitudelatitude
For Atlanta: latitude = 33o 45’ N longitude = 84o 23’ W
Terrestrial Coordinates
t1
t2
t3
Lunar Months
All motions are counterclockwise
Time from t1 to t2 is the “sidereal month” (This is time required for realignment with respect to the stars and equals 27.3 days.)
Time from t1 to t3 is the “synodic month”(This is the time between repetition of phasesand equals about 29.5 days. This is what weuse for our calendar.)
Earth
noonmidnight
sunset
sunrise
to the Sun
sunlight
sunlight
Lunar Phases
fullmoon
newmoon
firstquarter
thirdquarter
waningcrescent
waxingcrescent
waxinggibbous
waninggibbous
Questions About Lunar Phases
• What is the time interval between new and full moons?
• What time does the full moon culminate?
• What time does the new moon culminate?
• What time does the new moon rise?
• What is the phase of the moon that culminates at sunset?
• What is the phase of the rising moon at sunset?
• What is the phase of the setting moon at midnight?
two weeks
midnight
noon
sunrise
first quarter
full
first quarter
Earth
to the Sun
sunlight
sunlight
“Earthshine”waxing
crescent
MoonSunlight reflected off day lit side of Earthilluminates dark part of crescent moon.
The effect is most obvious just before andjust after new moon
The Tides
Earth
Moon
1. Imagine a perfectly spherical Earth uniformly flooded by an ocean.
2. The presence of the Moon produces a gravitational attraction on the Earth whose strength varies
inversely with distance from the Moon.
3. The water on the near side of the Moon is pulled away from
Earth, raising a high tide.
4. On the far side, the Earth is effectively pulled Moonward away from the water, yielding another high tide.
Moon Factoids
• A “blue moon” is when more than one full moon occurs in the same calendar month.
• The moon undergoes “synchronous” rotation and revolution (i.e. the periods of rotation and revolution are identical), so one side of the moon always faces the Earth.
• The “harvest moon” involves the rising of the full moon in late September and early October. Due to the angular tilt of the moon’s orbital plane with that of the Earth, the bright moon appears to rise at about the same time in the early evening when the moon is full at the time of the “autumnal equinox”.
Moon Myths
• The phase of the moon has no effect on human behavior.
• There is no such thing as the “dark side of the moon.”
• We did indeed land humans on the moon in the six Apollo landings between July 1969 and December 1972.
• For more about “lunatics”, the “moon hoax” and other astronomical pseudoscience, see:
www.astrosociety.org/education/resources/pseudobib.html
Tilt of Earth’s Spin Axis
23.5o tilt
The Earth’s spin axis is tilted by 23.5 degrees off vertical with respect to the “ecliptic plane” (plane of the Earth’s orbit around the sun)
The spin axis remains essentially parallel to itself during the course of the year
sunlight
sunlight
Summer Solstice – 21 June
equator
antarcticcircle
arcticcircle
tropic ofCancer
sunlight
sunlight
Winter Solstice – 21 December
equator
antarcticcircle
arcticcircle
tropic ofCapricorn
tropic ofCancer
sunlight
sunlight
Vernal Equinox – 21 March
equator
antarcticcircle
arcticcircle
tropic ofCancer
tropic ofCapricorn
sunlight
sunlight
Autumnal Equinox – 21 September
equator
antarcticcircle
arcticcircle
tropic ofCancer
tropic ofCapricorn
The Culminating Sun
• The sun culminates in the zenith (i.e. straight overhead) at noon for observers located on the tropic of Cancer (latitude = 23.5o N) on the day of the summer solstice.
• The sun culminates in the zenith at noon for observers located on the tropic of Capricorn (latitude = 23.5o S) on the day of the winter solstice.
• The sun culminates in the zenith (i.e. straight overhead) at noon for observers located on the equator (latitude = 0o) on the days of the equinoxes.
Other Seasonal Extremes
• The sun never rises for observers north of the arctic circle on the day of the winter solstice
• The above conditions are reversed for the antarctic circle.
• The sun moves 360o around the horizon for observers located at the north and south poles on the days of the equinoxes.
• The sun never sets for observers north of the arctic circle on the day of the summer solstice
Tilt of Earth’s Spin Axis
NS
W
Ewintersolstice
summersolstice
equinox
The sun rises on the east point and sets on the west point on the days of the equinoxes, givingequal periods of “day” and “night”.
The sun is in the sky for the longest duration on the summer solstice and illuminatesthe northern hemisphere most directly.
Temperature Effect
• Summer days are longer and the sun is more intense (due to the more direct illumination angle). Thus summer is hotter than winter.
• If the Earth’s spin axis were not tilted by some angle, we would have no seasons.
• There is a lag of the seasons when comparing the dates of the solstices with the actual extremes in temperature because it takes time to heat up the oceans and atmosphere at the onset of summer and to cool them off at the onset of winter.
Precession of Earth’s Spin Axis
1. The rotation of the Earth distorts it into an “oblate” spheroid flattened
at the poles
2. Moon’s orbital plane is tilted by 5o from our equator
3. Moon’s gravitational pull on Earth attempts to pull
bulge into lunar orbital plane
23.5o
4. Earth responds to this pull by slowly “precessing” its spin axis around a circle in the sky
once every 26,000 years
5. Spin axis now points to Polaris. 13,000 years from now,
Vega will be our “pole star”
Sun
Shadows and Eclipses
Penumbra
Umbra Earth
Eclipse of the Moon (Lunar Eclipse)
Earth’s Orbit
Moon’s Orbit
Occurs at Full Phase when Moon is also at the “line of nodes” of its orbitwith respect to the ecliptic
A lunar eclipse lasts for many hours and can be seen from the majority of theEarth’s surface
Eclipse of the Sun (Solar Eclipse)
Occurs at New Phase when Moon is also at the “line of nodes” of its orbitwith respect to the ecliptic
A solar eclipse lasts for only for a few minutes and can only be seen from very restrictedlocations on the Earth’s surface
Earth’s Orbit
Moon’s Orbit
Total and Annular Eclipses
Vertex of Umbral shadow is at or below Earth’s surface, so a total eclipse is possible
Vertex of Umbral shadow is above Earth’s surface, so only an annular eclipse is possible
Example of a Solar Eclipse Path
See Richard Monk’swebpage on eclipses:www.williams.edu/astronomy/IAU_eclipses/
Total Solar Eclipse of 21 June 2001 from Zimbabwe
See Richard Monk’swebpage on eclipses:www.williams.edu/astronomy/IAU_eclipses/
Bailey’s Beads
Solar Corona
“Diamond Ring”
Upcoming Lunar and Solar Eclipses
Solar Eclipses:
15 Jan 2009 (annular) – Asia & Africa 11 July 2010 (total) – South Pacific Ocean 4 Jan 2011 (partial) – Europe, Africa & central Asia 1 Jun 2011 (partial) – east Asia, far N. America, Iceland
1 Jul 2011 (partial) – south Indian Ocean25 Nov 2011 (annular) – south Africa, Antactica, Tasmania, New Zealand
The next total solar eclipses visible from the U.S. will be on 21 Aug 2017 and 8 Apr 2024. The 2017 eclipse will be visible from Georgia.
Lunar Eclipses:
31 Dec 2009 – not U.S.15 Jan 2010 (annular) – not U.S.26 Jun 2010 (partial) – not U.S.21 Dec 2010 (total) – all U.S.15 Jun 2011 (total) – not U.S.10 Dec 2011 (total) – all U.S.
Motions of the “Wanderers” – The Planets
The night sky facing south
WestEast
normal “direct” (eastward) motion retrograde motion
Mars
The Geocentric Explanation
stationary Earth
+ .deferentcenter
deferent
equant
epicycle
All motions are circular
Epicycle moves at constant angular rate about the equant
Adjustable parameters include diameters of epicycle & deferent, distance of equant from deferent center, and rates of motion along epicycle & deferent
Developed in detail around 140 AD by Claudius Ptolemy and very successfully used for 1500 years!
direct
retrograde
Mars
The Heleocentric Explanation
Sun
Earth Mars
direct motion
direct motion
retrograde motionaround “opposition”
First proposed in detail by Nicolaus Copernicus in ~1505 but not published until De Revolutionibus in 1543.
Oppositions of Mars
“Favorable”Opposition
Earth is at aphelion
closest approach is 34 million miles
Mars is at perihelion
“Unfavorable” Opposition
Earth is at perihelion
closest approach is 68 million miles
Mars is at aphelion
Oppositions of Mars occur at 26-month intervalsOn 27 Aug 2003, Mars had its most favorable opposition in 73,000 years
Earth
Maximum EasternElongation
Maximum WesternElongation
InferiorConjunction
SuperiorConjunction
Orbital Configurations for an Inferior Planet
Sun
Opposition
Eastern Quadrature
Western Quadrature
Earth Conjunction
Orbital Configurations for a Superior Planet
Sun
Giants of the Heliocentric Theory
• Tycho Brahe (1546-1601) – Greatest pre-telescopic observer, produced extensive observations of Mars that were critical to proving the Heliocentric Theory.
• Johannes Kepler (1571-1630) – Hired as Tycho’s assistant but only gained access to Tycho’s complete data after Tycho’s premature death. Kepler discovered three “laws of planetary motion” that revolutionized the understanding of the solar system.
• Galileo Galilei (1564-1642) – First used the telescope for observing the night sky in 1609. His discoveries were monumental and included proof of the Heliocentric Theory.
• Isaac Newton (1642-1727) – Developed the Law of Universal Gravitation and three laws of motion that completely explain Kepler’s Laws of Planetary Motion.
• Nicolaus Copernicus (1473-1543) – Developed the Heliocentric Theory but waited until just before his death to release his great book, De Revolutionibus.
Kepler’s First Law
Planets revolve around the sun in elliptical orbits with the sun located at one focus of the ellipse
+focus focuscenter
sun
planet
Kepler’s Second Law
The line from the sun to a planet sweeps out equal areas in equal time intervals.
t1
t2areaA
t3
t4
areaBareaA = areaB if t2-t1 = t4-t3
perihelion(fastest)
aphelion(slowest)
Kepler’s Third Law
For any two planets, the ratio of their mean distance from the sun cubed equals the ratio of their orbital periods squared.
(D1/D2)3 = (P1/P2)2
Planets far from the sun take longer to orbit the sun than do planets nearer the sun.
Galileo’s Telescopic Discoveries
• Lunar Feature – Found the moon to have craters, mountains and complicated terrain. He also reported spots on the sun, although it turns out they had first been reported centuries earlier by Chinese astronomers.
• Satellites of Jupiter – Discovered four large moons of Jupiter (still often referred as the “Galilean satellites”) which clearly orbited Jupiter and contradicted the geocentric premise that all bodies move around the Earth.
• Rings of Saturn – Galileo reported that Saturn had “ears” as his telescopes couldn’t quite make out the true nature of the rings.
• Phases of Venus – He discovered that Venus exhibited a complete cycle of phases, which it could not do under the constraints of the geocentric theory. This was proof of the heliocentric theory.
• New Stars – Discovered that his telescopes revealed far more stars than are accessible to the unaided eye.