Unit 1
Cycles of the Sky
Astronomy vs. Astrology
Astronomy: The study of the Universe
Astrology: The belief that the movement of celestial bodies affect life on Earth
Universal Forces
Strongest
Weakest
Electromagnetic forces are associated with charged particles
Electric and magnetic forces are the only forces that can both attract and repel
Coulomb (Electromagnetic)
Strong Nuclear
Strong nuclear force acts only on the neutrons and protons in the nucleus of an atom to hold them together
Acts only over an extremely small distance (10-15 meters)
Force is 100 times stronger than the electric force of repulsion at these distances
Weak Nuclear
An attractive force that acts only over a distance of 10-18 meters
This force is related to radioactive decay
Gravity
An attractive force between two objects
Strength of this force changes according to the mass of the objects and their distance from each other
Greater the mass of an object, the greater the force of gravity.
The closer two objects are to one another, the greater the force of gravity.
Gravity (cont.)
This force is the weakest but is the most effective over long distances, therefore it dominates on an astronomical scale
Matter and energy are equivalent (They are two forms of the same thing)
The universe is made up of matter consisting of elements, atoms, and subatomic particles
Each division up the line indicates an increase in size by 100,000
The Scales of The Universe
The range of objects we study are from the extremely small subatomic particles, to objects which are gigantic, such as a galaxy or the size of the known universe itself.
Distances in the Universe
1.) Astronomical Unit (AU): The mean or average distance from the Earth to the sun
About 93 million miles
Typically used to measure distances within our solar system
2.) Light year: The distance light can travel in one year
In a vacuum light travels at about 186,000 miles per second
It take 8 minutes and 20 seconds for sunlight to reach the Earth
It is 4.2 light years to Proxima Centauri, the closest star to us excluding the sun
This is about 5.88 trillion miles in one year
3.) Parsec: the distance at which an observer sees the maximum angle between the Sun and the Earth to be one arc second
Equal to about 3.26 light years
So where are we in the big picture?
The Earth is neither central nor special
It was formed nearly 4.6 billion years ago and is located near a medium sized main sequence star
The Earth is part of our solar system, which consists of the sun and the eight planets surrounding the sun
The solar system has no clear boundary
The “edge” of the Solar System is the area between the Kuiper Belt and the Oort Cloud.
Kuiper Belt – an area of space extending from the orbit of Neptune (about 30 AU’s) to about 50 AU’s from the Sun.
The Kuiper Belt has remnants from when the Solar System formed and objects here are composed of mainly frozen ices (methane, ammonia, and water).
Short-period comets (take less than 200 years to orbit the Sun) typically originate in the Kuiper Belt.
The Kuiper Belt also contains some dwarf planets such as Pluto, Haumea, and Makemake.
Oort Cloud – A spherical region of space outside of the Kuiper Belt. It extends from about 5000 AU to 100,000 AU.
It is believed that most long-period comets come from the Oort Cloud.
Our solar system is part of the Milky Way Galaxy
A galaxy is a massive system of stars held together by their mutual gravity
The Milky Way:
1. Has several hundred billion stars
2. Is about 100,000 light years across
3. Orbits the galactic core once about every 240
million years
Aka -Local Group
The Local Group is a collection of more than 50 galaxies (Including the Milky Way and Andromeda Galaxies)
It has a diameter of about 10 million light years.
There is about 2 million light years between the Milky Way and Andromeda galaxies.
Our Supercluster – Laniakea (Video)
100 million light years across
Our address in the universe!
A cluster of superclusters 200 million light years across
The Great Attractor
Article
Videos
All this is a part of our universe!
Celestial Sphere
Celestial Sphere – an imaginary sphere surrounding the Earth on which on objects in the sky can be located
The celestial sphere is a model only and does not necessarily match physical reality
The immense distances to stars prevents us from sensing their true 3-D arrangement
Naked eye observations (What we see on the celestial sphere) treats all stars as if they are at the same distance
Celestial horizon – An imaginary circle on the celestial sphere whose plane is at a 90° to the zenith and nadir.
You could also say it is the point where the sky meets the ground or the horizon.
There is an imaginary “rod” or axis on which heavenly bodies turn – The north and south poles are the end of this “rod”
Equator – divides earth into north and south halves
Prime Meridian – divides the earth into east and west sides
The imaginary lines on the globe are projected onto the celestial sphere above them
North celestial pole
South celestial pole
Celestial Equator
Zenith – The point on the celestial sphere that lies directly overhead at your location
We define the location of an object on the celestial sphere using the coordinates of declination and right ascension
Declination is comparable to latitude on the Earth
Right ascension is comparable to longitude on Earth
The 0 mark is the celestial equator and the reading can be negative
The 0 mark is the vernal equinox and divided into 24 equal sections
Red lines mark declination
Blue lines mark right ascension
0° declination
0 hour of right ascension
Betelgeuse
D = 7° 24’ 25”
RA = 5h 55m 10.3 s
15° declination
-15° declination
Motions of the Earth
1. Rotation – When Earth spins on its axis
Responsible for daily cycles
2. Revolution – When Earth goes around the sun
Responsible for yearly cycles
3. Precession – When Earth wobbles on its axis
24 different time zones & the International Date Line
Leap year, seasons, eclipses, certain meteor showers, etc.
Rotation
1 rotation (or spin) of the Earth on its axis = 1 day
The Earth’s rotation is responsible for the daily cyclic motions such as the apparent westward motion of the Sun, Moon, and stars across the sky each day.
Earth rotates in a counter clockwise motion
Rotation (continued)
The Sun, Moon, and stars rise in the East and move across the sky to set in the West
Circumpolar Stars or stars near the celestial poles do not rise or set but move in a circular motion around the poles
The celestial poles appear stationary
Circumpolar stars
Australia
Non circumpolar stars
Western Europe
Time Lapse Photos
Revolution
1 revolution of the Earth around the sun = 365.25 days
Over four different years the 0.25 days are combined to make one extra full day which we add to our calendars as February 29th. This is called a leap year.
Our movement around the sun causes a given star to rise 3 minutes and 56 seconds earlier each night
Revolution (continued)
Orbit – The specific path in space followed by a celestial
We have an elliptical orbit, NOT a circular orbit
body (comparable to revolution)
Aphelion – the point in an orbit where a body is farthest from the Sun
Perihelion – the point in an orbit closest to the Sun
Ecliptic – The line the Sun traces across the celestial sphere
The name arose because only when the new or full moon crosses this line can an eclipse occur
Because Earth revolves around the Sun, it changes our view of the stars
From our perspective, the Sun appears to move through the stars along a special path called the ecliptic.
From an outside view, we see the Earth revolve around the Sun. We define the plane of the Earth’s orbit as the ecliptic plane.
September
March
We can see how different stars appear at different times of the day by looking at the position of the Sun against the backdrop of stars. The side of the Earth facing the Sun is experiencing “day,” while the side of the Earth turned away from the Sun is experiencing “night.”
Constellations – Fixed arrangement of stars that resemble animals, objects, or mythological figures
Zodiac constellations are star patterns located along the ecliptic path.
While astrology uses 12 signs, astronomy recognizes 13 actual constellations along the ecliptic.
The 13 constellations of the zodiac
Constellations of the Zodiac
Seasons and The Ecliptic
The Earth is tilted on its axis at a 23.5° angle
This tilt causes the seasons
The tilt also causes the ecliptic not to be aligned with the celestial equator. This means the altitude of the sun in the sky will change throughout the year.
The angle of the Earth causes the Earth to be heated unequally
Solstices (Northern Hemisphere)
Solstice – When the direct rays of the Sun are farthest North or South of the Equator
Summer Solstice (June 21st) – Sun’s direct rays are over the Tropic of Cancer (Long days, short nights)
Winter Solstice (December 21st) – Sun’s direct rays are over the Tropic of Capricorn (Short days, long nights)
Equinoxes (Northern Hemisphere)
Equinox – When the direct rays of the Sun are over the Equator (the number of hours of daylight and number of hours of night are approximately equal)
Spring Equinox (March 21st ) – Sun’s direct rays are over the Equator and heading North
Fall Equinox (September 21st) – Sun’s direct rays are over the Equator and heading South
The Sun’s altitude in the sky changes throughout the year
Not only will the Sun’s altitude change, but the location where it meets the horizon will change
Precession
Precession – the slow conical (top-like) motion of the Earth’s axis of rotation
It takes 26,000 years for the Earth to complete one cycle of precession
The gravitational forces of the Sun and the Moon pulling on the Earth as it rotates causes the Earth to undergo a top-like motion called precession. Over a period of 26,000 years, the Earth’s rotation axis slowly moves in a circular motion.
This precession causes the position of the North Celestial Pole to slowly change over time.
Today, the North Celestial Pole is near the star Polaris, which we call the “North Star.”
In 3000 BC, Thuban was close to the North Celestial Pole.
In 14,000 AD, Vega will be in this location.
Phases of the Moon
Lunar (Latin origin) – of or relating to the Moon
The Moon goes through a complete set of phases every 29.5 days
Why is the first and third quarter phases have that name when only half of the Moon is lighted?
The Moon will rise about 49 minutes later each night.
Things to think about
Why do we not have an eclipse each month?
Which is more common, a solar eclipse or lunar eclipse?
During a solar eclipse, what phase is the moon in?
During a lunar eclipse, what phase is the moon in?
How far apart are a lunar and solar eclipse?
The Moon’s orbit is at a little more than a 5° tilt from the Ecliptic plane
An annular solar eclipse happens when the Moon is far enough away that the shadow does not reach the Earth.
Annular Solar Eclipse
Total Solar Eclipse