AST101

Scale of the solar system

100 billion stars

roughly in the milky way

Light year

Distance light travels in a year 9.461 trillion km

300,000km/s

The farther away you look, the farther back in time you look

Light takes time to travel

• can look at the past far away but cant see present

• can look at present on earth but not the past

There are 1 trillion galaxies visible from Earth!

Can’t see any more because its too far (beginning of universe)

• 1 trillion galaxies each with roughly 100 billion stars

The cosmic web

Galaxy clusters and webs

The universe

• Further we look, further back in time

• Universe is expanding and cooling, expand faster further away

• Universe is now transparent, but it wasnt 13.7 billion years ago

Primodial plasma

• Hot, dense gas, glowing

• Doppler shifted to microwaves now

• Cant see further past

• Universe HAS a beginning

Science is an epistemological system

• Objectively describes and predicts the behviour of the universe

• Is testable

Orbit Summary

• All orbit in the same plane (Ecliptic Plane)

• All orbit the same direction, sun rotates in the same direction too

• All have elliptical orbits.

• Closer the planet is to the sun, the faster it orbits

All = including dwarf planets

Mercury

• Closest to sun

• No atmosphere 

• Highly cratered surface

• Rocky exterior with iron core 

• Temp = -170C to 425C (day/night) HOTTTTTTT

• No moons

• Two probes

  • Mariner 10

  • Messenger

Venus

• About same size as earth

• Lots of volcanoes

• THICK CO2 atmosphere

  • Runaway greenhouse effect due to this

  • Always 460C everywhere 

• It rains acid 

• No moons 

• Rotates very slowly and backwards 

• Many probes and landers

Earth (tilt?)

• About the same size as venus

• Lots of volcanoes

• O2 + N2 atmosphere

• Large oceans regulate CO2

• Surface heavily affected by life 

• Large moon

• 23.4 degree rotational tilt producing significant seasons

Mars

• Extinct volcanoes

• Very thin CO2 atmosphere

• Oceans have evaporated

• No life

• Two tiny moons

• Polar ice/dry-ice (frozen CO2)

• 25.2 degree orbital tilt produces significant seasons

Terrestrial planets summary

• Small

• Rocky

• Relatively thin or no atmosphere

• Few moons

• Made from heavy elements

Jupiter

• Largest planet

• Thick gaseous atmosphere surrounds a giant ball of liquid hydrogen

• Very faint rings

• Many moons: more than 60 known to date

• Several probes have visited

Saturn

• Second largest planet 

• Structure is much like jupiter

• Giant rings 

• Many moons: more than 60 known to date

• Several probes

Uranus

• Coldest planet

• Small rocky core (we think)

• Thick water + ammonia + methane mantle

• H2/helium atmosphere

• Rotation axis is tilted 98 degrees

• Thin rings and lots of moons

Neptune

• Furthest planet from the sun

• Very similar to uranus

• Rotation axis 28 degrees

• More surface features

• Strongest winds in the solar system: up to 2100km/h

Jovian planets overview

• Large

• Gas + liquid

• Many moons

• Mostly light elements 

Earth’s moon

• Rocky

• No atmosphere

• Cratered

• No geological activity

• Looks similar to mercury

• Orbit plane is tipped a bit to the eliptical plane

Jupiter’s moons

• Io, Europa, Ganymede, Callisto (over 1000km diameter)

• over 60 known

• High ice content 

• Io is the most geologically active in the solar system (volcanoooooooes)

• Orbit plane is close to elliptical plane

Saturn's moons

• over 60 known

• 5 are over 1000km 

• High ice content

• Orbit in the same plane and direction as the rings

Uranus’ Moons

• over 60 known

• 4 over 1000km

• High ice content

• Orbit in same plane as uranus’ rotation -97 degrees

Neptune’s moons

• 14 known 

• Only 1 over 1000km – triton

  • Orbit not the same plane or direction as neptune’s rotation

  • Surface is frozen nitrogen

Dwarf planets

• 9 identified, but may be hundreds

• Very dim, hard to see

• Only pluto and ceres have been visited by probes

• Pluto has very high ice content (lots of frozen nitrogen)

Asteroid belt

• Around a million asteroids larger than 1km diameter

• Total mass is around 3% of the moons

• Some asteroids have been visited by probes 

• Between mars and jupiter 

• Some between jupiter, some after jupiter

25143 Itokawa

• One of millions of smaller asteroids

• 330m length

• Visited by the Hayabusa space probe (Japan) in 2005

• Samples returned to earth for study.

Kuiper belt

• Past neptune’s orbit

• Vast collection of small icy bodies

• Include several dwarf planets

• Total mass around the earth’s moon

• Two objects visited by New Horizons Probe

Arrokoth

Kuiper Belt object visited by the New Horizons probe in 2019

36 × 20 × 10 km in size

6 light-hours from Earth

Comets

• Icy bodies from outer solar system

• Highly elliptical orbits

• Emit tails when they get close to the sun

Voyager Probes

In 1977, alignment of planets was just right to visit all 4 jovian planets

• used each planet’s gravity to accelerate and change direction to next planet (slingshot)

• two launched — voyager 1 & 2

• still operating, furthest man made object, 20 light-hours from earth

Heading directions

e.g. W34N

or 304 degrees

Seasons summer vs winter

Summer:

• Daylight is longer and darkness is shorter

• Sun gets “higher” in the sky, so sunlight is more direct

• So Hotter!

.

Winter:

• Daylight is shorter and darkness is longer

• Sun does not get as “high” in the sky, so sunlight is at an angle

• So Colder!

Seasons — cont.

Summer

• june 21

• sun reaches 70 degrees above horizon

• 15 hour 26 mins of daylight

.

winter

• december 21

• sun reaches 23 degrees above horizon

• 8 hour 55 min of sunlight

December solstice

Northern hemisphere (winter solstice)

• tilted furthest away from the sun

• shortest day/longest night

.

Southern hemisphere (summer solstice)

• tilted furthest towards the sun

• longest day/shortest night

June solstice

Northern hemisphere (summer solstice)

• tilted furthest toward the sun

• ongest day/shortest night

.

Southern hemisphere (winter solstice)

• tilted furthest away from the sun

• shortest day/longest nightl

The poles in the June and Dec solstice

Artic circle

• constant daylight in June solstice

• constant nighttime in Dec solstice

.

South pole

• constant daylight in Dec solstice

• constant nighttime in June solstice

Spring and fall equinox

Sun shines equally on both hemispheres, vertical sun at equator

.

Spring equinox

• sun setting in south pole

• sun rising in north pole

.

fall equinox

• sun setting in north pole

• sun rising in south pole

The angle light hits a surface matters!

Not hotter because closer to the sun in summer!

Because:

• Light at a steep angle is concentrated — brighter, hotter

• Light at shallow angle is spread out — dimmer, colder

• (also longer day — more time to heat up)

Constellations

• Looking at the sky, one can see patterns in the stars

• We can imagine lines between them

• Diff places diff constellations

Astrology

Western popular astrology claims to be able to determine personality based on stars and planets at time of birth

• did double blind study

• no correlation found

• “confirmation bias”

Moon phases and eclipses

• Sun illuminates one side of the moon

• The other side is in shadow

• The phase we see depends on where the moon is compared to the sun

• Phase repeat each time the moon orbits the earth once

• Always see same face of the moon

.

• Lunar eclipse happens when the moon enters the earth's shadow, NOT the cause of phases

• Solar eclipse happens when the moon blocks the sun

Moon orbiting in the ecliptic

The moons orbit is tipped up by 5 degrees, stays constant

• passes below earth’s shadow

• passes above sun

• until it hits the NODE

  • middle of orbit intersects ecliptic plane — crosses the ecliptic

  • if lines up with earth and the sun = eclipse

  • twice a year hit the node

Moon phases — new moon

• Moon is lined up with the sun — Earth, moon, sun 

• Tipped = no eclipse

• Far side of the moon (never see) is lit up — Unlit side faces the earth

• The only time a solar eclipse can happen 

• Transits (is highest in the sky) when the sun does

• Rises the same time as the sun rises and sets the same time as the sun sets

• Not generally visible

Moon phases — waxing(growing) crescent

• 3-4 days after new moon

• Transit (highest in the sky) 3 hours after the sun 

  • rises 3 hours after sun, sets 3 hours after sun

• Most of the time its in the daytime, but can see it in the night 

• Easiest to see right after sunset 

(*depends on daylight saving time (rises and sets (daylight length)/2 before and after transit, 3 hours only true around the equinox)

1st quarter moon

• 1 week after the new moon

• Moon is lit from the side

• Transits 6 hours after the sun 

• Rise around 6 hours after the sun, set around 6 hours after the sun 

• Noon till midnight 

• Easiest to see at night before midnight

Waxing gibbous

• 10-11 days after the new moon 

• Transits 9 hours after sun does

• Rise 9 hours after sun

• Sets 9 hours after sun

• Easiest to see at night - may set before sunrise

Full moon

• Two weeks after the new moon

• Only time a lunar eclipse can happen

• Transits 12 hours after the sun does 

• Rises around 12 hours after the sun rises (so – near sunset)

• Sets around 12 hours after the sun sets (so – near sunrise)

• Visible most of the night

Waning (decreasing) gibbous

• 10-11 days before the next new moon

• Transits around 9 hours before the sun does

• Rises around 3 hours* after the sun sets

• Sets around 3 hours* after the sun rises

• Rises before midnight, and is up the rest of the night and morning

3rd Quarter

• ~ week before next new moon

• transits 6 hours before sun transits

• rise 6 hour before sun rises, sets 6 hours before sun sets

• rises around midnight, up for rest of night and morning

Waning crescent

• 2-4 days before new moon

• transits 3 hours before sun transits

• rise 3 hours before sun, sets 3 hours before sun

• easiest to see before sunrise

Solar eclipse

• When the moon blocks light from the sun

• Happens when new moon crosses ecliptic plane while in front of the sun

• Because moons orbit is tipped relative to the ecliptic plane, happens rarely (every few years, somewhere on earth)

• Only casts a shadow on a small part of earth

How does solar eclipse work?

• Apparent size of the moon is the same as the size of the sun (Sun is about 400 times wider than Moon, but about 400 times farther away from Earth)

• North and south, angle of seeing the moon changes, see partial eclipse

• Inside umbra – eclipse

• Outside – partial

Annular eclipse

Type of solar eclipse where Moon passes directly in front of the Sun but appears too small to completely cover it

• due to slight changes in orbit

Lunar Eclipse

• when moon enters the Earth’s shadow

• happens when full moon crosses ecliptic plane

• can be seen from anywhere on earth

• more common than solar eclipse (earth is larger than moon, larger shadow)

• “red” due to light interacting with atmosphere

Problems with my model/simulation:

• not to scale

• tilt of lunar orbit is dramaticized

• moon’s rotation axis should be tipped

• orbits should be ellipses, not circles

• positions of everything is affected by everything else in the solar system

• orbits should not be centred on sun and earth

• speeds should change depending on distance

• orbital plane of moon should precess around every 18.6 years

Astronomy before copernicus — observations

• Sun, moon and stars rise and set every day

• Earth feels fixed 

• Stars don't seem to change 

  • Move across the sky over the year, but constellations never change 

• Things fall to earth

Astronomy before copernicus — assumptions

• Came from ancient greeks 

• The “heavens” are perfect and incorruptible 

• Circles are perfect – heavens are perfect

Astronomy before copernicus — conclusions

• The earth is the center of the universe 

• Up there is all the other stuff 

• Everything revolves in circles around the earth

  • Perfect things must go in circles

The Geocentric Universe

• Earth is centre of universe

• All celestial objects revolve around the earth

Matter of perspective

Problems — Retrograde Motion

• Planets move from night to night relative to the stars

• Dont follow uniform pattern relative to the stars

• Appear to turn around relative to the stars

• Called retrograde motion

Cycles and epicycles

• Perfect circles dont work with retrograde motion

• Conclusion: planets move in circles within circles

• Retrograde is consistent with a circles within circles pattern

Copernicus

• 1473–1543

• Catholic canon/scholar

• Diplomat, Economist, Translator 

• A different idea

  • The sun is the center of the universe

  • The planets go around the sun 

  • Moon orbits the earth

Apparent Retrograde Motion

• Earth orbits faster than Mars

• As earth passes mars, the position of mars compared to background stars change

• Explains the motions of planets on the sky

Tycho Brahe 1546 - 1601

• Danish nobleman, Had his own island as an observatory, Was exiled over politics

.

• Made extremely precise accurate measurements of the motion of mars

• Observed super nova (a star suddenly appearing in the sky, then disappearing)

  • Heavens incorruptible?

• Used parallax to constrain distances 

• From his data, current models clearly had problems – copernicus’s model and geocentric model doesnt work

Johannes Kepler 1571 - 1630

• worked from Brahe’s data

• concerned with inaccuracy of current models for theological reasons

• Discovered a fix

Kepler’s First Law

The orbit of each planet about the Sun is an ellipse with the Sun at one focus

• Eccentricity: distance between 2 focuses

• E = 0, circle

Kepler’s Second Law

A planet moves faster in the part of its orbit nearer the Sun and slower when farther from the Sun, sweeping out equal areas in equal times

Kepler’s Third Law

More distant planets orbit the Sun at slower average speeds, obeying a precise mathematical relationship

Galileo Galilei 1564 - 1642

• Pioneer in telescopes

• Experimented with laws of motion and gravity

• Using telescopes he observed:

  • Moons has craters

  • Jupiter has moons

  • Venus has phases

Galileo’s observations of the moon

Has mountains and craters

So:

• not everything in heavens are perfect circles and spheres

• maybe elliptical orbits are fine

Jovian Moons

Galileo observed 4 moons orbiting Jupiter

So:

• not everything orbits earth!

Phases of Venus

Through his telescope, Galileo observed that Venus has phases

• apparent size of venus changes throughout the year

• “new” phase happens when venus appears largest

• “full” phase happens when venus is in line with the sun (and smallest)

.

So:

• venus not orbiting earth

• has to be orbiting the sun!