Lesson 1: The Size of the Universe, Constellations and Angular Sizes, Seasons, The Moon

Question 1

What is the cosmic web?

Answer 1

(the purple spider web image)
the image shows a large-scale structure of our universe

• Each point of light represents mass, each individual pixel is a galaxy (each purple pixel)
• Sections of the universe where there are no galaxies (voids) and sections where galaxies are close together

Question 2

What is our ‘cosmic address’ ?

Answer 2

Earth - Solar System - Milky Way Galaxy - Local Group - Local Supercluster

• Local Group: local galaxies to us (gravitationally bound together that are orbiting each other)

Question 3

Provide a very brief description of the following objects: star, planet, natural satellite, asteroid, comet, solar (star) system, nebula, galaxy, the universe

Answer 3

Star: A large, glowing ball of gas that generates heat and light through fusion; are super dense

Planet: Can be rocky, icy, gas; orbits a star

Natural Satellite (aka the moon): An object that orbits a planet

Asteriod: A small and rocky object that orbits a star, found in the asteriod belt (between Mars and Jupiter)

Comets: A small and icy object that orbits a star; orginates further from star; as it gets closer to the star, it creates a tail and starts melting

Solar (Star) System: A star and all the material that orbits it, including planets and moons

Nebula: An interstellar cloud of gas and/or dust; thousands of satrs within nebula because of a lot of material in one place

Galaxy: A great island of stars in space, all held together by gravity and orbiting a common centre

Universe (the observable Universe): 14 billion years old; the sum of all matter and energy; everything within and between all the galaxies

Question 4

What is the speed of light? Since light travels at a finite speed, what does this mean for our view out into the universe?

Answer 4

Light travels at a finite speed (300,000 km/s)

Takes 1 second of time for light to travel from Moon to Earth; 8 mins from Earth to Sun
* We see the Sun how it is 8 minutes ago… The farther away we look, the further back in time we look

Question 5

What is the definition of a light year?

Answer 5

The distance that light can travel in 1 year
* can travel 10 trillion km in a year

Question 6

What big benefit is there to the
idea of look back time?

Answer 6

we’ll be able to know more about the history of the universe

Question 7

How big is the universe (either in terms of look back time or distance)?

Answer 7

Can see light as far as 14 billion years (age of universe)

Question 8

How do our lifetimes compare to the age of the universe? (hint: maybe think of the ‘cosmic calendar’ example from class)

Answer 8

Cosmic calendar: a scale of which we compress the history of the universe into 1 year
* our lifetimes are small compared to the extensive history of the universe

Question 9

What are the basic motions of Earth in space? How fast are we moving around the Sun? How fast is our solar system moving around the Galaxy?

Answer 9

Earth:
*Earth rotates around its axis once everyday
* takes earth a 365 days to orbit the Sun (counter-clockwise)

Solar system:
* Orbits Milky Way every 230 million years; constantly moving

Question 10

What is an Astronomical Unit?

Answer 10

Astronomical Units (AU) - distance from Earth to the Sun

Question 11

What is prograde v. retrograde spin/rotation?

Answer 11

Prograde: (most) planets who spin counter-clockwise

Retrograde spin: planets who spin clock-wise ex. Venus

Question 12

How are galaxies moving within the Universe?

Answer 12

Galaxies are carried along with the expansion of the universe
* we live in an expanding universe

Question 13

How many stars can the naked eye see? Are those stars part of the Milky Way or not?

Answer 13

With the naked eye, we can see about 3,000 stars as well as the Milky Way

Question 14

What is a constellation?

Answer 14

Is a region of the sky
* There are 88 official constellations that fill the entire sky

Question 15

What is the celestial sphere? North celestial Pole? South celestial pole? Celestial Equator? Ecliptic? What is the significance of the Zodiac constellations?

Answer 15

Celestial sphere: Stars that all circle around earth in a ‘sphere’

North and South celestial poles are the two points in the sky where Earth’s axis of rotation intersects the celestial sphere
* north celestial pole points at polaris star

The celestial equator is a line that runs on the same plane as the equator of Earth

Elliptic: path that the sun takes on our sky (yellow line), even the solar system (Neptune, Jupiter, etc.) follow the elliptic

Zodiac constellations help us map where stars are in the sky

Question 16

In a dark sky we are able to see the band of light we call the “Milky Way;” explain what it is we’re looking at with reference to the shape of the Milky Way.

Answer 16

• Milky way is a spiral galaxy
• 100 billion stars in the milky way

Question 17

What does altitude and direction mean? How do we define them on our sky?

Answer 17

An object’s altitude (above horizon) and direction (along the horizon)
* Direction - point me in the direction where I need to face
* Altitude - how high should I look up (max 90°)

Question 18

What is the Zenith? Horizon? Meridian? Can you define their altitudes or directions?

Answer 18

Zenith: the point directly above you (90°)

Horizon: line(s) where earth’s surface and the sky meet (0°)

Meridian: Line passing through zenith and connecting N and S points on horizon (180°)

Question 19

How do you use your hands to measure angular sizes on the sky?

Answer 19

Start at 0° at the horizon (direct your body where the star is)
point your fist (10°) to the horizon
and measure up till you reach the atlitude of the star

Question 20

What is the angular size of full circle? What is that in arcminutes? in arcseconds?

Answer 20

Full circle = 360°
1° = 60’ (arcminutes)
1’ = 60 (arcseconds)

Question 21

What’s the difference between the angular size and physical size of an object on the sky (ie, the angular size of the Moon is , the physical size of the Moon is ?)

Answer 21

the further away an object is, the smaller it appears.
* We call this ‘apparent size’ / angular size of an object - because you’re not actually measuring the object’s true size, you’re measuring the angle it takes up in your vision

Question 22

Why do the constellations we see depend on BOTH the observer’s latitude (location on the planet) and the time of year?

Answer 22

Stars differ from season to season…
* Because the sky that we look at, depends on latitude and the time of year we are around the sun

If the observer is in the northern hemisphere, they can see Polaris; if the observer is beyond the equator, they will not see northern stars

Question 23

What are latitude and longitude?

Answer 23

Latitude: position north or south of equator
Longitude: position east or west of prime meridian
* Prime Merdian - Greenwich, England

Question 24

What is the relationship between the altitude on the sky of the celestial pole to a local observer and the local observer’s latitude on the planet?

Answer 24

The sky varies with latitude but not longitude
* If the observer is in the northern hemisphere, they can see Polaris; if the observer is beyond the equator, they will not see northern stars

Question 25

What is the difference between a solar day and a sidereal day? why?

Answer 25

Solar day = 24 hours
* time it takes for the Earth to rotate about its axis so that the Sun appears in the same position in the sky

Sidereal Day (Earth’s rotation period) = 23 hours, 56 minutes * * * the time taken by the earth to rotate on its axis relative to the stars

Question 26

Why do we have seasons? Explain how seasons work, and compare how this works in the northern hemisphere vs southern hemisphere?

Answer 26

The reason we have seasons because of earth’s tilt and earth’s movement around the Sun and the Sun’s directness

• In the winter the Earth’s Northern Hemisphere is tilted away the sun
• In the summer the Earth’s Northern Hemisphere is tilted towards the Sun
• (vice versa with the Sountern Hemisphere)

Question 27

How do we mark the changing of the seasons? And what are the significances of these days throughout the year?

Answer 27

We mark using:
* Summer solstice
* Winter solstice
* Spring (vernal) equinox
* Fall (autumnal) equinox

We can recognize solstices and equinoxes by the Sun’s ecliptic
* Summer Solstice: sun’s highest path in the sky,
* Winter solstice: sun’s lowest path in the sky
* Equinoxes: sun’s middle path in the sky
○ Such as Fall equinox and Spring equinox

Question 28

How does the orientation of Earth’s axis change with time?

Answer 28

Earth actually precesses (wobbles) over about 26,000 years
* Polaris won’t always be the North Star

Question 29

Explain the phases of the Moon. Why does the moon have phases? What are they? Be able to determine a rising and setting time for any phase.

How long is the Moon’s orbit period?

Answer 29

Lunar phases are a consequence of the Moon’s 27.3 day orbit around Earth
* Waning (shrinking on the left side), gibbous (illuminated)
Waxing (growing on the right side)
* First quarter is on the right

Waxing - afternoon/evening
Waning - late night/morning

Question 30

Why do we only see one side of the Moon from Earth’s perspective?

Answer 30

Synchronous rotation: The Moon rotates once with each orbit

Question 31

What’s the difference between a Solar and Lunar eclipse?

Answer 31

Solar eclipses can occur only at new moon
* The moon is dark and it is inbetween the Sun and Earth

Lunar Eclipse
* Lunar eclipses: can occur only at full moon
* Lunar eclipses can be penumbral, partial, or total (total is when the Moon looks red)
* When Earth is in between the Sun and Moon

Question 32

Why don’t we have an eclipse at every full and new moon?

Answer 32

The Moon’s orbit is tilted 5 degrees on the ecliptic plane
* So we have about two eclipse seasons each year, with a lunar eclipse at full moon and solar eclipse at new moon

Question 33

Moon: What’s the difference between a sidereal month and a solar month?

Answer 33

The Moon’s sidereal period—the period of its revolution about Earth measured with respect to the stars—is a little over 27 days
* Sidereal month: is 27.3217 days
* Solar month: the time interval in which the phases repeat—from the phases full to full

Question 34

What is a synchronous rotation? How does it work? What’s another name for this?

Answer 34

Synchronous rotation (captured rotation or tidal locking): describes the rotation of an object that always shows the same face to an planet that it is orbiting because its rotation period and orbital period are equal
* The Moon rotates on its axis in exactly the same time that it takes to revolve about Earth.
* the Moon always keeps the same face turned toward Earth

Question 35

What’s the difference between the umbra and penumbra?

Answer 35

The shadows of the Earth and Moon consist of two parts
1. Umbra: a cone where the shadow is darkest
2. Penumbra: more diffuse region of darkness (the moon is in a less concentarted shadow area)