ASTR1220 Midterm (night sky - gravity)

what unit of distance do we use

Kilometers!

inner planets of solar systems are…

Rocky (terrestrial) and small e.g. Venus, Mercury, Earth, Mars

Outer planets are…

gas giants (made of mostly hydrogen) e.g. Jupiter, Neptune, Uranus, Saturn

Astronomical Unit (AU)

Average distance between Earth and Sun. 1 AU = 150 mil km

Light Year

UNIT OF DISTANCE that light can travel in one year. Speed of light is always constant and light is the fastest thing in the universe. 1 ly = 9.5 trillion km

how long does it take light to travel to earth?

8 mins from the sun to get to us on earth. 1 AU = 8 light minutes

When we look at an object that's 1,000 ly away, we see it... 

As it was 1,000 years ago 

• Explanation: bc ly describes a unit of distance NOT time, but when sometimes is so so far away, time eventually gets involved because we don't see it instantly as it is not close. Something that is 1,000 ly away takes 1000 years to travel to Earth 

how do we measure the size of astronomical objects?

using trigonometry (do some practice problems with it)

earth rotates around its axis once a day

this gives us night and day

celestial sphere

the location of stars onto the surface and we view the celestial sphere form the inside unlike an Earth globe

North celestial pole

point in space directly over Earth’s north pole

altitude

how high in degrees above the horizon

why do the stars appear to move?

bc of Earth’s rotation. the stars themselves aren’t moving rather it’s Earth that’s rotating causing stars trail

polaris

north star that lies near the north celestial pole. you can see it in the northern hemisphere

in what direction does earth rotates? what direction do stars move?

counter clock wise from east to west. so all stars also move in arcs across the sky from east to west

how does astrology work?

The sun is in front of a zodiac sign each month (we can't see that zodiac in the night sky for that month until 6 months later when that constellation appear directly opposite of another constellation) 

+ e.g. In Jan, the sun is in front of Capricorn. 6 months later you'll see Capricorn directly on the other side of the axis 

ecliptic plane

earth’s orbital path around the sun

how do we get seasons?

earth’s rotational axis which is tilted 23.5 degrees in the ecliptic plane. the more extreme the tilt, the more extreme the seasons (90 degrees most extreme season & 0 degrees = no seasons bc every place on earth would receive the same amount of sunlight year round and everywhere except the poles would have the same 12 hours of day and night)

in the northern hemisphere, earth’s tilted rotational axis cause north pole to be pointed where in the summer? in winter?

polaris is pointed towards the sun in the summer. polaris is pointed away from the sun in the winter (helpful to draw it out to visualize)

define:

summer solstice

spring equinox

fall equinox

winter solstice

summer solstice: longest day of the year, we get the most daylight bc sun’s path is the highest on Earth and rise to the most extreme north of due east and west

spring equinox and fall equinox: 12 hours of day and night so that’s split evenly

winter solstice: shorter day of the year, we get the least daylight and the longest night bc sun’s path is the lowest on earth and set at the most extreme south of due east and west

(helpful to draw it out)

rank earth’s distance from the sun with respect to the seasons (in northern hemisphere)

1. closest to the sun= winter. earth is closest to the sun during the winter. in the northern hemisphere we get winter while the southern hemisphere enjoys summer due to earth’s rotational axis as polaris points away from the sun (in the northern hem)

2. Spring

3. Fall

4. furthest to the sun = summer. in the northern hemisphere earth is furthest from the sun in the summer bc polaris is pointed to the sun, we enjoy warmer season. the southern hemisphere would get winter 

rank how much time the person spends in daylight from most to least daylight based on Earth's tilt: 

1. 23.5 degrees tilt (facing towards the sun) 

2. 0 degree tilt 

3. 23.5 degrees tilt (facing away from the sun) 

4. 45 degrees tilt 

5. 90 degrees tilt --> experience least daylight where only half the world enjoying daylight and night all the time 

what happens when earth’s tilt is 90 degrees? 0 degrees?

90 degrees = Each hemisphere (north and south) would take turns pointing at the sun for half a year. 6 months of winter in the northern hemisphere and 6 months of summer in the southern hemisphere, vice versa. most extreme season

• uranus gets the most extreme season due to most tilt

0 degrees = no seasons. north and south hem would experience same amount of day and light

• jupiter gets no season due to having least tilt

 

What makes planets so special? 

• Move with respect to fixed stars 

• Change in brightness 

• Change speed 

• Some planets (Mars) appear to undergo retrograde motion 

heliocentric model

Earth is not the center, rather all planets revolve around the sun (theorized and developed by Copernicus; later supported by Galileo) 

define:

Speed

Velocity

Acceleration

speed- how fast an object moves over time. 

• speed = 100 miles / hr

velocity- involves speed and direction 

• velocity = 25 km/hr towards the northwest

acceleration- the changes in speed OR direction over time

• speed up from 0 to 60km/hr

• changing direction

• acceleration is caused by an applied force

Kepler’s laws

describe the patterns and relationships that predict the positions and motions of the planets

kepler’s 1st law

Orbit of each planet around the sun is an ellipse with the sun at one focus point. Circle has an eccentricity of 0 (e=0). the bigger the eccentricity the more elliptic it is.

• higher eccentricity = longer ellipse

• lower eccenticity = closer it appears to a circle

what is the eccentricity of earth?

close to zero (e=0) as earth’s orbit around the sun is close to a circle

kepler’s 2nd law

as planet moves around its orbit, it sweeps out equal areas in equal times. this means:

• as a planet orbit further from the sun, it travels slower; as planet orbit closer to the sun, it travels faster. but both sweeps out equal areas as they are all in the same orbit (helpful to draw it out)

kepler’s 3rd law

more distant planets orbit the sun at slower average speeds

• p^2 = a^3 

p=  orbital period (in years) 

a= avg. Distance from sun (in AU) 

This is an empirical relationship but it is not a real law 

• The size of the orbit determines the orbital period; planets that orbit near the sun orbit with shorter periods than planets that are far from the sun (e.g. Mars orbit sun slower so 1 year in Mars is longer than Earth which orbits the sun faster as it’s closer)

• the planet’s mass does not matter here bc it only depends on the object’s orbit’s distance from the sun

Does earth or mars have higher orbital velocity? 

Earth have higher orbital velocity because it is closer to the sun than Mars. Making Earth orbit the sun at a higher speed than Mars 

what is the explanation for Mars Retrograde motion? 

when one planet that goes fast (Earth) overtakes the slower planet (Mars) which makes it looks like Mars is "going backwards" but it's really Earth going faster and overtaking Mars in the orbital space due to Kepler's third law 

Specific explanation of Kepler's 2^nd and 3^rd law 

• 2^nd law tells us what one particular planet does when it orbits a star (sun) 

• 3^rd law tells how the orbital periods are related to the orbital distances for all the planets in the solar system 

  • Planets that are in an orbit located near the sun have short orbital periods 

  • Planets that are in an orbit located farther will have longer orbital periods 

According to Kepler's second law, a planet with an orbit like Earths would: 

Experience very little change in orbital speed over the course of the year 

If a small weather satellite and a large international space station are orbiting earth at the same altitude above earth's surface, what is true? 

• Each has the same orbital period. Why? Bc mass of the objects does not matter when it comes to determining whether their orbital period is longer/shorter. If they are orbiting earth at the same altitude above earth's surface, then that means they are all in the same orbital period 

 

define:

mass

density

size

weight 

• Mass: the amount of matter in an object 

• Density: mass/volume = density 

• Size: how much space the object takes up  

• Weight: gravitational force that acts upon an object's mass. You will have different weight on Earth vs Moon but same mass 

  • Mass DOES NOT CHANGE but weight does depending on gravity 

  • Weight of astronaut = 1200N of force on Earth; weight of astronaut = 200N on Moon 

 

Newton’s 1st law of motion

An object moves at constant velocity (or remains at rest) unless a force act on it to change its speed or direction 

• A hockey puck glides across the ice at constant speed until it hits something 

• An astronaut will coast in space along a straight line at constant speed (unless a force acts on it) 

Newton’s 2nd law of motion

Force = mass x acceleration

if mass goes down, then acceleration goes up.

if force goes up so does acceleration

• Throwing a football harder will make it go faster; given the same force, a smaller mass will be given more acceleration (bc it is lighter) 

Newton’s 3rd law of motion

For any force, there is always an equal and opposite reaction force 

• Sitting still, you're exerting the same amount of force on your chair as it's exerting on you. No motion means forces are balanced (equal and opposite)  

• A rocket is propelled upward by force equal and opposite to the force with which the gas is being expelled 

Newton's Universal Law of Gravity 

• Every mass attracts every other mass 

• Attraction is increases as the product of their masses (M1 * M2) 

• Attraction decreases as the square of the distance between their centers 

which laws are universal: Newton’s or Kepler’s?

Newton’s. Kepler’s describe a pattern or observation instead

make sure to do practice prob with universal law of gravitation!

is the force the earth exerts on you larger, smaller, or the same force you exert on it?

earth and I exert equal and opposite forces on each other