Astronomy 100 The Solar System Tuesday, Wednesday, Thursday Tom Burbine

What is a galaxy?

HW #2 Due tomorrow (June 10 th )

What is a galaxy? Is a massive, gravitationally bound system consisting of stars, gas and dust, and dark matter. Galaxies can contain between ten million and a trillion stars Dark matter is matter that does not emit or reflect enough radiation to be seen, but whose gravitation effects can be felt

When we are looking at stars or galaxies We are looking into the past Light-year is the distance light travels in a year.

Milky Way Galaxy Milky Way is 100,000 light years in diameter There are ~200 billion stars in the Milky Way (estimates from billions stars)

What is the Universe?

Sum total of all matter and energy – all galaxies and everything between them Observable universe – portion of the universe that can be seen from Earth, probably only tiny portion of the whole universe ~93 billion Light-years wide

What causes seasons? The tilt of the Earth’s axis relative to the ecliptic

Seasons

Solstices Summer Solstice –June 21 – Northern Hemisphere receives its most direct sunlight Winter Solstice – December 21 – Northern Hemisphere receives its least direct sunlight

Equinoxes Sun shines equally on both hemispheres Spring Equinox – March 21 – Northern Hemisphere goes from slightly tipped away from the Sun to slightly tipped towards Fall Equinox – September 21 - Northern Hemisphere goes from slightly tipped toward from the Sun to slightly tipped away

Why does the orbital difference not matter?

Reasons There is only a 3% difference in the distance from the Earth to the Sun at its farthest and closest point The Earth is actually closer to the Sun in the winter than in the summer

Mars is now visible in the sky stmhttp:// 369.stm

Angular size We measure distances in the sky using angles 180 o in the observable sky

More precise distances 1 degree = 60 arcminutes (symbol ´) 1 arcminute = 60 arcseconds (symbol ´´) So something that is 2 degrees, 10 arcminutes, 22 arcseconds would be written as 2 o 10´ 22´´

Terminology for looking at the sky

Celestial Sphere an imaginary sphere of infinite extent on which all celestial objects appear to lie

Celestial Sphere

What is a constellation?

Constellations People refer to constellations as a pattern of stars Astronomers refer to constellations as specific regions of the sky In 1928, the IAU (International Astronomical Union) decided there were 88 constellations Many of the constellation names go back thousands of years

Constellations The constellations are totally imaginary things that poets, farmers and astronomers have made up over the past 6,000 years (and probably even more!). The real purpose for the constellations is to help us tell which stars are which, nothing more.

What is this constellation?

Orion Bigger the star, the brighter it is

Orion was the son of the god of the sea, Poseidon and a great hunter. One story is that he made an enemy of Hera who sent a scorpion to sting him. Orion was restored to health by Ophiuchus, the first doctor of medicine. Another story is that Artemis was tricked by by Apollo to shoot an arrow at Orion. When he died, Poseidon asked Zeus to put him among the stars.

Ursa Major Ursa Major, the Great Bear, was identified with a bear by native American Indians of the Northeastern United States and the ancient Greeks. The name common in Britain, the Plough,seems to have a medieval origin, Another common name among northern European cultures is the Wain, a shortened form of wagon

What are the constellations named after 14 men and women 9 birds 2 insects 19 land animals 10 water creatures 2 centaurs one head of hair a serpent a dragon a flying horse a river 29 inanimate objects

Originally considered part of Leo’s tail

Named after Queen Berenice II of Egypt, wife of Ptolemy III Euergetes (246 BC BC) Around 243 BC, the king undertook a dangerous expedition against the Syrians, who had murdered his sister. Berenice swore to the goddess Aphrodite to sacrifice her famous long hair if her husband returned safely. He did, she had her hair cut, and placed it in the goddess' temple. By the next morning, the hair had disappeared. To appease the furious king and queen (and save the lives of the temple priests), the court astronomer, Conon, announced that the offering had so pleased the goddess that she had placed it in the sky. He indicated a cluster of stars that at the time were identified as Leo's tail, but now have been called Berenice's Hair.

Zodiac The zodiac is an imaginary belt in the heavens extending approximately 8 degrees on either side of the Sun's apparent path (the ecliptic), that includes the apparent paths of the Moon and the planets Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune.

Question: Why do all the planets seem to follow the same path?

Answer: The planets, the Earth, and the Sun all tend to fall in the same plane called the ecliptic

Why don’t all the constellations have ancient names?

Ancient cultures such as the Greeks and Egyptians could not see the constellations in the Southern Hemisphere

Question: Why is the path of the constellations on the zodiac not on the celestial equator?

Answer: The rotation axis of the Earth is inclined with respect to the ecliptic

Polaris is called the North Star Brightest star in the constellation Ursa Minor. 48th brightest star in the night sky It is very close to the north celestial pole, making it the current northern pole star. Polaris' altitude, or height above the horizon, is equal to an observer's latitude.

Long Term Changes

Precession Earth precesses like a top Precession - phenomenon by which the axis of a spinning object (e.g. a part of a gyroscope) "wobbles" when a torque is applied to it

Forces For a top, the force is gravity, which is trying to pull the top down For the Earth, the forces are due to the pull of the Sun and Moon, which is trying to align the Earth’s axis with the ecliptic

Because of precession The position of a star that corresponds to the North Celestial Pole changes

Models When you have a model of how something works, you should be able to predict what will happen If observations do not fit the model, either the observations or the model is wrong The ancient astronomers wanted to predict the positions of planets in the sky

What did the ancients think That the Earth was the center of the universe That the celestial sphere was rotating around the Earth However, there was two observations that caused problems with this idea –Apparent retrograde motion –Inability to detect stellar parallax

Greek model

Apparent Retrograde Motion = “backward” motion

Retrograde Motion

Retrograde Planet appears to go backwards in its orbit

Stellar Parallax Stellar Parallax – The apparent shift in the position of a nearby star (relative to distant objects) that occurs as we view the star from different positions in the Earth’s orbit of the Sun each year

The distance the star moves is greatly exaggerated in this figure. Stellar parallax can only be seen by a telescope.

Ancient astronomers could not detect stellar parallax If Earth orbited the Sun, ancient astronomers believed that they would see differences in angular separation of stars as the Earth rotated around the Sun Since they saw no changes in angular separation of the stars, they assumed the Earth was the center of the universe They could not fathom that stars are so far away that stellar parallax is undetectable by the human eye

Ptolemy’s ( AD) Model of the Universe

Nicholas Copernicus ( ) Copernicus came up with a model that the Earth revolves around the Sun Similar to what Aristarchus (310 – 230 BC) thought 2000 years before However, Copernicus’ models did not match observations since he wanted everything to arouind in perfect circles

Tycho Brahe ( ) Tycho Brahe was the greatest naked eye observer of all time He lived before the invention of the telescope His observations of the alignment of Jupiter and Saturn occurred two days later than when predicted by Copernicus Tycho came up with a model where the planets orbit the Sun but the Sun orbits Earth

Johannes Kepler ( ) Tried to match circular orbits to Tycho’s data Couldn’t do it Because Tycho’s observations were so good, Kepler had to come up with a new model

Kepler was trying to match an orbit to Tycho’s observations of Mars “If I believed that we could ignore these eight minutes of arc, I would have patched up my hypothesis accordingly. But, since it was not permissible to ignore, those 8 minutes pointed to the road to a complete reformation in astronomy.” Kepler came up with his 3 laws of planetary motion

Kepler’s 1 st Law The orbit of each planet about the Sun is an ellipse with the Sun at one focus (there is nothing at the other focus)

Differences between ellipses and circles

Eccentricity (e) e = distance between the two foci/length of major axis e of circle is 0 The larger e becomes, the more eccentric the orbit

Definitions Perihelion – planet closest to the Sun Aphelion – planet farthest from the sun Semi-major axis (a) – the average of a planet’s perihelion and aphelion distances

Kepler’s 2 nd law As a planet moves around its orbit, it sweeps out equal areas in equal times. This means that the planet travels faster when it is nearer the Sun and slower when it is farther from the Sun

Kepler’s 3 rd Law More distant planets orbit the Sun at slower average speeds, obeying the precise mathematical relationship p 2 = a 3 where p is a planet’s orbital period in years and a is the average distance from the Sun in astronomical units.

Calculations The period for the Earth to go around the Sun is 1 year The average distance of the Earth to the Sun is 1 Astronomical Unit

How long does it take Jupiter to go around the Sun If Jupiter is 5.2 Astronomical Units from the Sun, how long does it take Jupiter to go orbit the Sun once p 2 = a 3 = = p = √140.6 = 11.9 years

Another example Mercury is 0.4 Astronomical Units from the Sun. How long does it take Mercury to orbit the sun once? –A) 1 year –B) 3 months –C) 9 months –D) 5 years

The calculation p 2 = a 3 = = p = √0.064 = 0.25 years

An asteroid takes 8 years to go around the Sun How far is the asteroid away from the Sun? –A) 1 AU –B) 3 AU –C) 4 AU –D) 8 AU

The calculation a 3 = p 2 = 8 2 = 64 a = (64) 1/3 = 4 AU

You can calculate a planet’s orbital speed Since you know a planet’s orbital distance And you know its orbital time You can calculate a planet’s average orbital speed

Laboratory

harmony-of-the-worlds#x-0,vepisode,1,0http:// harmony-of-the-worlds#x-0,vepisode,1,0

Any Questions?