Discovering the Universe Astronomy and human culture have always been intertwined Astronomical events have defined the cycles of human life They have inspired great religion stories The scientific revolution started with efforts to explain planet motion in the sky And now science has given us the modern lifestyle we enjoy Lots of things we use every day come from space programs (for some examples, visit here) And in fact, we came from the sky we are star stuff we are made of elements made by stars and just as we came 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? Sometimes we see patterns even when there s nothing there 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 Sometimes we see patterns even when there s nothing there When there is something there, we sometimes identify it as something that it isn t 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 Sometimes we see patterns even when there s nothing there 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 or even actually being some mythological character or other So patterns of stars have been explained as being somehow associated with or even 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
Back to the traditional meaning (which most people mean when they use the word) The stars in a constellation are typically different distances from Earth But they don t appear to be They appear to be fixed to the inside surface of a sphere surrounding us (a small section of which is visible above) The modern constellations completely cover the surface of this celestial sphere Most ancient people thought the celestial sphere was real, because it looks that way We know now that the celestial sphere doesn t really exist, but we use the concept anyway The Celestial Sphere The celestial sphere has features strongly linked to Earth, which is at its center 4
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 The Celestial Sphere The ecliptic is the apparent path of the Sun against the background of the stars 5
The Celestial Sphere Celestial coordinates are analogous to latitude and longitude The Celestial Sphere Declination (Dec) ~ latitude Dec = 0 at the celestial equator The Celestial Sphere Right ascension (RA) ~ longitude RA = 0 h in Pisces, which is where the Sun is at the Spring equinox 6
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 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 7
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 and the planets are in when you are born is supposed to affect your fate But is this possible, according to our best current scientific understanding? Does astrology work? One strike against it is that your sign is likely not what you thought it was How can this be? 8
It is due to the difference between sidereal years and tropical years The standard calendar is based on the seasons First Why do you think we have seasons? Seasons are caused by the tilt of the Earth, not by how close the Earth is to the Sun It s summer when the rays of the Sun hit the Earth s surface more directly, and it s winter when the rays hit more at a glancing angle The solstices and the equinoxes define the first day of each season Our calendar is based on the tropical year, which is measured from one vernal (Spring) equinox to the next But the problem with this is that like any spinning body including this top the Earth precesses This makes a difference between the length of a tropical year and a solar year Notice the direction of precession for the Earth 9
The tropical year of our calendar measures from one vernal equinox to the next But the Earth precesses a little during each sidereal year The result is that a tropical year is about 20 minutes shorter than a sidereal year And also that the vernal equinox position shifts against the background constellations against the zodiac Compound that 20 min/year 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? Where is it really? We ll find out on the next slide 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 10
It could be that the horoscopes are wrong, but that celestial bodies really can influence terrestrial events But this is not consistent with our scientific understanding... 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 11
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 Stars in the Local Sky You see any stars that pass through the half of the celestial sphere centered on your zenith Most 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 Even though they traveled only limited distances, the ancient Greeks saw that stars are different 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 The brightest object in the night sky is not a star, but? It is the Moon And it looks different at different times 12
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 Even as it goes through phases, the Moon always presents the same face to us 13
Even as it goes through phases, the Moon always presents the same face to us That does not mean that it does not rotate 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 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 14
The Moon, and the Sun, have been humanity's constant companions The Moon, and the Sun, have been humanity's constant companions The Sun rises and sets everyday, as usual 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 15
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 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 Eclipses occur when the Sun, Earth, and Moon are in a line 16
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 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 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? 17
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 18
Eclipses can only happen when the Moon is in the ecliptic plane Which is when it is in locations called nodes And that can only happen during eclipse seasons Eclipse seasons are due to the Moon s angular momentum You might expect eclipse seasons to be 6 months apart 19
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 Imagine the node line is precessing as the Earth orbits the Sun 20
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 and some other kind 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 Eclipses can occur only when 21
Eclipses can occur only when the Moon is at a node Eclipses can occur only when the Moon is at a node and 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 22
Two kinds of eclipses: Two kinds of eclipses: Moon between Earth and Sun Two kinds of eclipses: Moon between Earth and Sun Solar 23
Two kinds of eclipses: Moon between Earth and Sun Solar Earth between Moon and Sun Two kinds of eclipses: Moon between Earth and Sun Solar Earth between Moon and Sun Lunar 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 24
Lunar eclipse types Penumbral Partial Total Solar eclipse types Total Partial Annular These are photos of an annular eclipse on Monday, 26 Jan 2009, taken in Bandar Lampung, Sumatra, Indonesia 25
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! 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 For those who wanted to impress and advance, this would have been an important tool 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? 26
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 27