Discovering the Universe Astronomy and human culture have always been intertwined Astronomical events day and night, seasons -- have defined the rhythms of human life They have inspired great myths and religion stories Attempts to explain planet motion birthed modern science Science has given us the modern lifestyle we enjoy (and many techy things are spinoffs from space programs) We ourselves what we re made of came from the sky from stars we are star stuff made of elements made by stars and just as we came ultimately from the sky much of our civilization has come from our drive to explain what we see in the sky Among the things we see in the sky are the patterns of stars associated with constellations A natural human tendency is to try to explain patterns 1
Among the things we see in the sky are the patterns of stars associated with constellations A natural human tendency is to try to explain patterns do you see a pattern here? There isn t one But sometimes we see patterns even when there isn t one A natural human tendency is to try to explain patterns When there is something there, we sometimes identify it as something that it isn t A natural human tendency is to try to explain patterns When there is something there, we sometimes identify it as something that it isn t This is called pareidolia 2
So patterns of stars have been explained as being somehow associated with oreven actually being some mythological character or other The pattern resembling a character like Orion is what most people mean when they use the word constellation So patterns of stars have been explained as being somehow associated with oreven actually being some mythological character or other The pattern resembling a character like Orion is what most people mean when they use the word constellation But today s astronomers use the word constellation somewhat differently Their constellations are regions of the sky containing the traditional constellation patterns 88 of them in all The modern constellations completely cover the sky, like continents and oceans completely cover the surface of the Earth 3
But back to the traditional meaning of constellation the stars in a constellation are typically different distances from Earth But they appear to be fixed to the inside surface of a sphere surrounding us (a small section of which is visible above) Of course, we know now they are not actually fixed They are actually moving very rapidly, if you recall But they are so far away that we can t detect their motion Most ancient people thought the celestial sphere was real, because it looks that way They did not realize that the patterns of stars are temporary The stars just happen to have moved to positions that make the patterns we see But given enough time, those patterns will change as the stars continue to move #constellationproject We know now that the celestial sphere isn t real, but we use the concept anyway 4
The Celestial Sphere The celestial sphere has features strongly linked to Earth, which is at its center The Celestial Sphere It has a north celestial pole directly above Earth's north pole The Celestial Sphere It has a celestial equator, which is an extension of Earth's equator 5
The Celestial Sphere The ecliptic is the apparent path of the Sun against the background of the stars The Celestial Sphere Celestial coordinates are analogous to latitude and longitude The Celestial Sphere Declination (Dec) ~ latitude Dec = 0 at the celestial equator 6
The Celestial Sphere Right ascension (RA) ~ longitude RA = 0 h in Pisces, which is where the Sun is at the Spring equinox The Celestial Sphere Stars, but not planets, always have the same Dec and RA Of course, over very long times, a star s Dec and RA will change The motion of the stars in the sky could be because the celestial sphere is turning or because the Earth is rotating We now know that it s the Earth rotating (a Foucault pendulum shows this, for example) But ancient people thought it was the rotation of the celestial sphere because they had no evidence otherwise, and it looks and feels that way Let s consider something else ancient people believed 7
Astrology Astrology is the belief that celestial bodies can somehow influence earthly events and human destinies To understand astrology, you need to understand sidereal rotation, why there are seasons, and how our calendar works Sidereal Day vs Solar Day A sidereal day is the length of time it takes for the Earth (or any celestial body) to turn once around its axis relative to the stars Such a full 360 rotation relative to the stars is called a sidereal rotation A solar day is the length of time it takes for the Earth to turn around far enough that the Sun is at the same location in the sky say at noon The Earth has to turn a little bit farther than one sidereal rotation to put the Sun in the same place in the sky So the solar day on Earth (exactly 24 h) is about 4 minutes longer than the sidereal day (23h 56 ) Any type of rotational motion spinning, orbiting, etc can have sidereal periods different from other types of periods, as in the case of Earth s sidereal vs solar day And this has implications for astrology, as you will see Here are the Sun, the Earth, and the ecliptic, the path the Sun takes through the constellations on the celestial sphere Do you recognize these constellations? They are the constellations of the zodiac, the astrological signs According to astrology, the sign the Sun, planets, and other celestial bodies are in when you are born is supposed to affect your fate 8
But is this possible, according to our best current scientific understanding? Does astrology work? One strike against it is that your horoscope sign is likely not what you thought it was How can this be? It is due to the difference between sidereal years and tropical years and how they relate to the seasons But first you need to understand why we have seasons Why do we have seasons? a) We have seasons because sometimes Earth is closer to the Sun than others. b) We have seasons because of the tilt of Earth s axis. c) We have seasons because of the Sun s sunspot cycle. 9
Seasons are caused by the tilt of the Earth It s summer when the rays of the Sun are more direct are more concentrated It s winter when the rays are less direct less concentrated The solstices and the equinoxes define the first day of each season Our calendar is based on the tropical year from one vernal (Spring) equinox to the next The problem for your horoscope sign is that Earth precesses Anything that spins in a gravity field does this This makes a difference between the length of a tropical year and a sidereal year Notice the direction of precession for the Earth (opposite its rotation direction) The tropical year of our calendar goes from one vernal equinox to the next But the tropical year is about 20 minutes shorter than a sidereal year This makes the vernal equinox position shift against the background zodiac constellations 10
The standard newspaper horoscope signs were set by Ptolemy ~2000 years ago Compound the 20 min/year precession effect over 2000 years, and you find that in 2000 years, the Earth has precessed the equivalent of ~1 month The result is that the Sun is not in the same constellation as it used to be So what is your sign, and where is the Sun supposed to be on your birthday? Adjusted birthday ranges for the various Sun signs Capricorn - Jan 20 to Feb 16 Aquarius - Feb 16 to Mar 11 Pisces - Mar 11 to Apr 18 Aries - Apr 18 to May 13 Taurus - May 13 to Jun 21 Is your sign what you thought it was? Gemini - Jun 21 to Jul 20 A. Yes Cancer - Jul 20 to Aug 10 Leo - Aug 10 to Sep 16 Virgo - Sep 16 to Oct 30 Libra - Oct 30 to Nov 23 Scorpius - Nov 23 to Nov 29 Ophiuchus - Nov 29 to Dec 17 Sagittarius - Dec 17 to Jan 20 B. No It could be that the horoscope tables are wrong, but that celestial bodies really do influence terrestrial events But this is not consistent with our scientific understanding... 11
For astrology to work we need a long-range effect some sort of force field We know of only four physical forces in our universe: strong weak electromagnetic gravity The only long-range physical force that could mediate the influence of planets on a child at birth is gravity The strength of gravity coming from the obstetrician that delivered you is comparable to the strength from a planet Object (planets are at closest distance) Ratio gravity object to gravity of 100 kg human at distance of 0.5 m Earth 367,000,000 Sun 230,000 Moon 1,410 Mercury 0.1 Venus 8.3 Mars 0.5 Jupiter 13.7 Saturn 1.0 Proxima Centauri 0.0000004 3 story 55 m x 35 m building at distance of 120 m 0.5 100-kg human at distance of 0.5 m 1.0 So the specific surroundings at your birth have similar effects to celestial bodies 12
Stars in the Local Sky You see any stars that pass through the half of the celestial sphere centered on your zenith Most of those stars rise (in the east) and set (in the west) But some stars circumpolar stars never set And other stars never rise Which ones these are for you depends on where you are on the globe Stars in the Local Sky The ancient Greeks traveled only limited distances, but they noticed that different stars are visible at different latitudes This led them to suggest that the Earth was not flat, and that the sky was not a simple dome inverted over it It also vouches for their powers of observation, which we ll come back to later Stars in the Local Sky The brightest object in the night sky is not a star What is it? It is the Moon And it looks different at different times 13
The Phases of the Moon The Phases of the Moon Main Entry: gib bous Pronunciation: 'ji-b&s, 'gi- Function: adjective Etymology: Middle English, from Middle French gibbeux, from Late Latin gibbosus humpbacked, from Latin gibbus hump Date: 14th century 1 a : marked by convexity or swelling b of the moon or a planet : seen with more than half but not all of the apparent disk illuminated 2 : having a hump : HUMPBACKED The Phases of the Moon Even as it goes through phases, the Moon always presents the same face to us 14
The Phases of the Moon Even as it goes through phases, the Moon always presents the same face to us That does not mean that it does not rotate The Phases of the Moon Even as it goes through phases, the Moon always presents the same face to us That does not mean that it does not rotate It does mean that from the Moon, the Earth hangs motionless in the sky The Phases of the Moon Even as it goes through phases, the Moon always presents the same face to us That does not mean that it does not rotate It does mean that from the Moon, the Earth hangs motionless in the sky, and goes through phases that are opposite to the Moon phases seen from Earth 15
The Phases of the Moon The Moon, and the Sun, have been humanity's constant companions The Phases of the Moon The Moon, and the Sun, have been humanity's constant companions The Sun rises and sets everyday, as usual The Phases of the Moon The Moon, and the Sun, have been humanity's constant companions The Sun rises and sets everyday, as usual The Moon goes through its phases every month, as usual 16
The Phases of the Moon The Moon, and the Sun, have been humanity's constant companions The Sun rises and sets everyday, as usual The Moon goes through its phases every month, as usual Usually... The Phases of the Moon The Moon, and the Sun, have been humanity's constant companions The Sun rises and sets everyday, as usual The Moon goes through its phases every month, as usual Usually... but when there are eclipses, things are different The Phases of the Moon Eclipses occur when the Sun, Earth, and Moon are in a line 17
The Phases of the Moon Eclipses occur when the Sun, Earth, and Moon are in a line The figure above suggests that eclipses should occur every month at new moon and at full moon The Phases of the Moon Eclipses occur when the Sun, Earth, and Moon are in a line The figure above suggests that eclipses should occur every month at new moon and at full moon But that doesn't happen The Phases of the Moon Eclipses occur when the Sun, Earth, and Moon are in a line The figure above suggests that eclipses should occur every month at new moon and at full moon But that doesn't happen Why? 18
The Moon s orbit is tilted ~5 with respect to the ecliptic plane Eclipses can only happen when the Moon is in the ecliptic plane Eclipses can only happen when the Moon is in the ecliptic plane Which is when it is in locations called nodes 19
Eclipses can only happen when the Moon is in the ecliptic plane Which is when it is in locations called nodes And those nodes need to be aligned with Earth and the Sun Eclipses can only happen when the Moon is in the ecliptic plane Which is when it is in locations called nodes And those nodes need to be aligned with Earth and the Sun That can only happen during eclipse seasons Eclipse seasons are due to the Moon s angular momentum It keeps the tilt almost the same during orbit 20
You might expect eclipse seasons to be 6 months apart You might expect eclipse seasons to be 6 months apart But they are slightly less than that, because the Moon s orbit precesses After half a year the approximate length of time between eclipse seasons Hold the angle of the nodes after 6 months in your mind 21
Imagine the node line is precessing as the Earth orbits the Sun Imagine the node line is precessing as the Earth orbits the Sun Precession causes eclipse seasons to be separated by slightly less than 180 days This is another example of a sidereal motion relative to the stars combined with orbital motion So to summarize The Moon's orbit is inclined to the ecliptic ~5 So most of the time Moon is above or below the ecliptic plane Only when the orbital nodes are aligned with Earth and Sun can eclipses occur about every 180 days 22
Eclipses can occur only when the Moon is at a node, and when it is new or full When it s new (Moon between Earth and Sun) Solar eclipse When it s full (Earth between Moon and Sun) Lunar eclipse Both types of eclipses involve shadows that have two parts: Umbra Penumbra This is due to Sun not being a point source As a result of the two-part shadow, both lunar and solar eclipses come in several types Lunar eclipse types Penumbral Partial Total 23
Solar eclipse types Total Partial Annular These are photos of an annular eclipse on Monday, 26 Jan 2009, taken in Bandar Lampung, Sumatra, Indonesia Solar eclipses are much harder to experience than lunar eclipses To see a lunar eclipse, it just needs to be night (and clear) when the eclipse happens To see a solar eclipse, you have to be in the right place at the right time because the umbra of a solar eclipse is only about 100 miles wide, and it moves at ~1700 km/h across the Earth! 24
Nearly 3000 years ago the ancient Chaldeans, a Babylonian dynasty, detected a regularity in the occurrence of lunar eclipses They found they recurred every 18 years, 11 1/3 days This is now known as the saros cycle An important tool for those who wanted to impress But it didn't always work Can you imagine why? Because of the 1/3 day Because of that, even though an eclipse will occur with regularity It will not always occur at the same place This is a potential problem for both lunar and solar eclipses But it is particularly a problem for solar eclipses Why isn t it such a problem for lunar eclipses? But it is particularly a problem for solar eclipses Why isn t it such a problem for lunar eclipses? Because they can be seen over a much larger region 25