ASTRO 101
Principles of Astronomy
Instructor: Jerome A. Orosz (rhymes with “boris”)Contact:
•Telephone: 594-7118
•Office: Physics 241, hours T TH 3:30-5:00
Text:“Discovering the Essential Universe,Fifth Edition”byNeil F. Comins
Course WWW Page
Note the underline: … ast101_fall2012.html …
Also check out Nick Strobel’s Astronomy Notes:
Next:
The Moon
The Moon
•The moon is relativelysmall:
–Mass = 1/81 of Earth’s
–Radius = 1/4 of Earth’s
–Gravity = 1/6 of Earth’s
•It appears large in the skybecause it is so close,about 240,000 miles,compared to 93,000,000miles to the Sun.
The Moon
•The albedo of the Moon is less than 10%.
The Moon
•The albedo of the Moon is less than 10%.
•The surface is composed of very dark rock(as dark as coal).
The Moon
•The albedo of the Moon is less than 10%.
•The surface is composed of very dark rock(as dark as coal).
•But why is the Moon so bright?
The Moon
•The albedo of the Moon is less than 10%.
•The surface is composed of very dark rock(as dark as coal).
•But why is the Moon so bright?
•The Moon is nearby, and we view itcontrasted against a dark sky.
The Moon
•The surface gravity on the Moon is 1/6 ofthe Earth’s.
The Moon
•The surface gravity on the Moon is 1/6 ofthe Earth’s. This is too weak to retain anatmosphere.
The Moon
•The surface gravity on the Moon is 1/6 ofthe Earth’s. This is too weak to retain anatmosphere.
•There is no water on the Moon, nor is thereweather of any kind.
The Moon
•The surface gravity on the Moon is 1/6 ofthe Earth’s. This is too weak to retain anatmosphere.
•There is no water on the Moon, nor is thereweather of any kind. The surface featureswe see are very old.
What do we see on the Moon?
•We always see basically the same side of the Moonfacing us, although there is some wobble.
What do we see on the Moon?
•The rotational period is equal to the orbital period.
What do we see on the Moon?
•Craters and mountains have been seen since thetime of Galileo. We know they are craters bylooking at the illumination patterns.
What do we see on the Moon?
•Craters are best seenduring the crescentphases since surfacefeatures cast shadows.
What do we see on the Moon?
•More craters as seenfrom a NASA probe.
How did the Craters Form?
•The craters on theMoon were caused byimpacts of largebodies, not byvolcanoes.
•In many cases you cansee trails of lighter-colored matter thrownout by the impacts.
How did the Craters Form?
•The lunar craters are caused by impacts, and not byvolcanoes.
What do we see on the Moon?
•We also see largedarker areas called“maria” (Italian for“seas”).
What do we see on the Moon?
•We also see largedarker areas called“maria” (Italian for“seas”).
What do we see on the Moon?
•We also see largedarker areas called“maria” (Italian for“seas”).
How did the Maria Form?
•An impact of a very large body and subsequent lavaflows may have formed the maria.
The Lunar Interior
•The inside of the Moon can bestudied using seismic equipmentleft by the Apollo astronauts,and by tracking the orbits ofspacecraft near the moon.
•The moon has an uneveninterior, with massconcentrations near the maria.
•The interior is cold, and thereare no active volcanoes today.
The Earth and Moon
•The Earth has anatmosphere
•The Earth is hotinside
•The Earth has lotsof water
•The Earth has lotsof iron
•The Moon does nothave an atmosphere
•The Moon is coldinside
•The Moon has verylittle water
•The Moon has littleiron
Next
•The formation of the Moon
The Formation of the Moon
•The density of the Moon is about 3.3grams/cc, somewhat less than the Earth.
The Formation of the Moon
•The density of the Moon is about 3.3grams/cc, somewhat less than the Earth.
This density is similar to the density of theEarth’s Mantle.
The Formation of the Moon
•The density of the Moon is about 3.3grams/cc, somewhat less than the Earth.
This density is similar to the density of theEarth’s Mantle.
The Moon is deficient in iron.
The Formation of the Moon
•The density of the Moon is about 3.3grams/cc, somewhat less than the Earth.
This density is similar to the density of theEarth’s Mantle.
The Moon is deficient in iron.
The surface composition of the Moon issimilar, but not exactly like that of theEarth.
The Formation of the Moon
•In the late 1960s, there were three maintheories on how the Moon formed:
The Formation of the Moon
•In the late 1960s, there were three maintheories on how the Moon formed:
1)Fission. The Earth somehow spun up and flungoff the Moon, leaving behind a depression thatwould become the Pacific Ocean.
The Formation of the Moon
•In the late 1960s, there were three maintheories on how the Moon formed:
1)Fission. The Earth somehow spun up and flungoff the Moon, leaving behind a depression thatwould become the Pacific Ocean.
2)The Moon was captured by the Earth’s gravity.
The Formation of the Moon
•In the late 1960s, there were three maintheories on how the Moon formed:
1)Fission. The Earth somehow spun up and flungoff the Moon, leaving behind a depression thatwould become the Pacific Ocean.
2)The Moon was captured by the Earth’s gravity.
3)The Moon was formed nearby the Earth at thesame time.
The Formation of the Moon
•In the late 1960s, there were three maintheories on how the Moon formed:
1)Fission. The Earth somehow spun up and flungoff the Moon, leaving behind a depression thatwould become the Pacific Ocean.
2)The Moon was captured by the Earth’s gravity.
3)The Moon was formed nearby the Earth at thesame time.
•All three ideas are probably not correct.
The Formation of the Moon
•The currently most accepted model is theejection of material caused by a giantimpact.
The Formation of the Moon
•The currently most accepted model is theejection of material caused by a giantimpact.
•A large body (perhaps bigger than Mars)collided with the young Earth and ejected aconsiderable amount of material from theEarth’s upper layers.
The Formation of the Moon
•The currently most accepted model is theejection of material caused by a giantimpact.
•A large body (perhaps bigger than Mars)collided with the young Earth and ejected aconsiderable amount of material from theEarth’s upper layers.
•This material condensed and formed theMoon.
The Formation of the Moon
•Computer simulations and chemical analysis of Moonrocks supports the collision/ejection theory.
The Formation of the Moon
•Computer simulations and chemical analysis of Moonrocks supports the collision/ejection theory.
•This collision could have caused the Earth’s rotationaxis to become tilted.
Newton’s Laws and Tides
•If the tides are causedby the Moon pulling onthe oceans, then why isthere usually two hightides per day?
Newton’s Laws and Tides
•If the tides are causedby the Moon pulling onthe oceans, then why isthere usually two hightides per day?
•Actually tides arecaused by differencesin the gravitationalforces.
Newton’s Laws and Tides
•Spring tides are whenthe Sun and Moon areroughly aligned (e.g.new and full moon).The tides tend to behigher at these times.
•Local conditions canalso effect the height ofthe tides.
Next:
Chapter 5: Other Planets andMoons.
Mercury
•Mercury is the closest planet to the Sun.
•It is never seen against a dark sky, and it is neverfar above the horizon.
Mercury as Seen From the Earth
•Here is the bestground-based imageof Mercury.
Mercury as Seen From Earth
•Mercury is hard to study from the ground since itis close to the Sun.
Mercury as Seen From Earth
•Mercury is hard to study from the ground since itis close to the Sun.
•We can measure its average density. We findthe density is 5.4 grams/cc, much like the Earth.
Mercury as Seen From Earth
•Mercury is hard to study from the ground since itis close to the Sun.
•We can measure its average density. We findthe density is 5.4 grams/cc, much like the Earth.
•We can measure the albedo, and we find it isabout 10%, much like the Moon.
Mercury as Seen From Earth
•Mercury is hard to study from the ground since itis close to the Sun.
•We can measure its average density. We findthe density is 5.4 grams/cc, much like the Earth.
•We can measure the albedo, and we find it isabout 10%, much like the Moon.
•Mercury mass is 5.5% of the Earth’s mass, andits gravity is 38% of the Earth’s.
Mercury as Seen From Earth
•Mercury is hard to study from the ground since itis close to the Sun.
•We can measure its average density. We findthe density is 5.4 grams/cc, much like the Earth.
•We can measure the albedo, and we find it isabout 10%, much like the Moon.
•Mercury mass is 5.5% of the Earth’s mass, andits gravity is 38% of the Earth’s.
•We expect Mercury to be similar to the Moon.
Mercury Seen up Close
•In 1974 NASA sent a probe to Mercury.
•It really does look like the Moon.
Mercury Seen up Close.
•Mercury is covered with craters.
Mercury Seen up Close
•In many cases you cansee rays of materialejected by the impacts.
Mercury’s Interior
•Mercury has a largeiron core.
•This core is relativelycold.
•There is very littleevidence of present-day geologicalactivity.
Mercury’s Interior
•Mercury has a large iron core.
•It is possible that a collision early in the history ofMercury could have stripped off less dense materialnear the surface, leaving behind the heavier material.
Mercury’s Rotation
•Since Mercury is so close to the Sun, tidal forceshave forced it into a 3-to-2 spin-orbit coupling.
•As a result, a day on Mercury is 2 Mercury yearslong!
Mercury
•Mercury has a very thin atmosphere.
•There is no water.
•There is essentially no erosion.
•It is relatively hot on the day side (up to 800oF)since it is near the Sun. However, on the nightside it can be as low as -280oF
•It looks a lot like the moon on the surface, but itis different in its interior.
Venus
•Venus has a mass and radius similar to thatof the Earth.
Venus
•Venus has a mass and radius similar to thatof the Earth. Its gravity is strong enough toretain a substantial atmosphere.
Venus
•Venus has a mass and radius similar to thatof the Earth. Its gravity is strong enough toretain a substantial atmosphere.
•The albedo is very high, more than 75%.
Venus
•Venus has a mass and radius similar to thatof the Earth. Its gravity is strong enough toretain a substantial atmosphere.
•The albedo is very high, more than 75%.
Venus
•Venus has a mass and radius similar to thatof the Earth. Its gravity is strong enough toretain a substantial atmosphere.
•The albedo is very high, more than 75%.We do not see the surface, but rather thetops of the clouds.
Venus
•Venus is the second closest planet to the Sun.
•It is never seen against a very dark sky, and it isnever far above the horizon.
Venus
•No surface featuresare seen from Earth.
Venus
•The cloud patterns arechanging over several hours.
Venus
•The surface temperature is about 475o C,compared to about 25o C for Earth.
Venus
•The temperature at the surface of Venus is high.
Venus
•The surface temperature is about 475o C,compared to about 25o C for Earth.
•The atmospheric pressure at the surface ofVenus is 90 times that of the Earth.
Venus
•The surface temperature is about 475o C,compared to about 25o C for Earth.
•The atmospheric pressure at the surface ofVenus is 90 times that of the Earth.
•The composition of the atmosphere is about96% CO2, compared to mostly N and O onthe Earth.
Venus
•The surface temperature is about 475o C,compared to about 25o C for Earth.
•The atmospheric pressure at the surface ofVenus is 90 times that of the Earth.
•The composition of the atmosphere is about96% CO2, compared to mostly N and O onthe Earth.
•??????
The Greenhouse Effect
•Venus has a “runaway” greenhouse effectthat heats the planet an extra 375o C.
The Greenhouse Effect
The Greenhouse Effect
•Venus has a “runaway” greenhouse effectthat heats the planet an extra 375o C.
•Some visible light from the Sun reaches thesurface and heats it.
The Greenhouse Effect
•Venus has a “runaway” greenhouse effectthat heats the planet an extra 375o C.
•Some visible light from the Sun reaches thesurface and heats it.
•The surface radiates the energy in theinfrared, which the CO2 in the atmosphereabsorbs.
The Greenhouse Effect
•Some visible light from the Sun reaches thesurface and heats it.
•The surface radiates the energy in theinfrared, which the CO2 in the atmosphereabsorbs.
•The extra heat “bakes out” more CO2 fromthe rocks.
The Greenhouse Effect
•The surface radiates the energy in theinfrared, which the CO2 in the atmosphereabsorbs.
•The extra heat “bakes out” more CO2 fromthe rocks.
•The extra CO2 leads to more trapping of thesurface infrared radiation.
The Greenhouse Effect
•The extra heat “bakes out” more CO2 fromthe rocks.
•The extra CO2 leads to more trapping of thesurface infrared radiation.
•The extra trapped heat bakes out more CO2,and so on…
The Surface of Venus
•Soviet spacecraft have landed on Venus andrecorded close-up pictures.
•These images show basalt, which is quite similarto lava rock.
The Surface of Venus
•The Venusian surface has been mapped with radarby the Magellan spacecraft.
•These maps reveal gently rolling hills, two“continents”, and many volcanoes.
The Surface of Venus
•The Venusian surface has been mapped with radarby the Magellan spacecraft.
•There are relatively few impact craters. Perhapsmelting of the surface has erased earlier craters.
Venus Summary
•Although Venus has a similar mass and radius asthe Earth, it is a very different place owing to therunaway greenhouse effect:
–The temperature at the surface is about 475 oC.
–The atmospheric pressure is about 90 times that onthe Earth.
–The atmosphere is mostly CO2.
NEXT:
Mars
•Named for the Roman god of war, owing toits red color.
•Its mass is 10% of the Earth’s mass, itsradius is 50% of Earth’s radius, and itsgravity is 38% of Earth’s gravity.
•Mars has usually been considered to be theplace to look for extraterrestrial life.
Mars
•In some ways, Mars is a bit like Earth:
Its rotation rate is 24 hours and 37 minutes.
The inclination of the axis is 25.2o.
The seasonal variations of the solar heating aresimilar to that on Earth.
Mars
•Mars is different from the Earth in someimportant ways:
The surface temperature ranges from about -125 oCto 25 oC, with typical temperatures below 0 oC.
The atmospheric pressure is about 1% of that on theEarth, which is too thin to maintain a significantgreenhouse effect.
Why the Interest in Mars?
•Every few years Mars passes relatively close toEarth.
•During these close passages, it is possible to seesurface features through a good telescope.
Early Observations of Mars
•Early observations were by eye (through a telescope).The observer had to draw what he saw.
•Here is drawing from 1800
Early Observations of Mars
•Here is drawing from 1877.
Early Observations of Mars
The Italian astronomer Schiaparelli made detailedobservations in the 1870s.
He noted the presence of “canali.”
Early Observations of Mars
•Schiaparelli meant “channels.”
•English translations used “canals,” implyingthat they are of intelligent design. Note,however, that beavers dig canals too.
Early Observations of Mars
•Schiaparelli meant “channels.”
•English translations used “canals,” implyingthat they are of intelligent design. Note,however, that beavers dig canals too.
•Near the turn of the 20th century, manypeople were convinced that there was lifeon Mars.
Life on Mars
•This famous moviemade in 1953 wasbased on H.G. Well’sclassic novel of 1898.
•On Oct 30, 1930,Orson Wells broadcasta fake news report ofinvading Martians,causing widespreadpanic.
Modern Views of Mars
•We know now that theearly observations of“canals” were in error.
•There are no cities orcivilizations.
