Some Facts about our Sun and other Stars

1.   NEVER STARE DIRECTLY AT THE SUN!

2.   Life on Earth would cease to exist without the Sun.  The Sun is the main source of
      energy for the Earth. It provides energy as light and heat. Plants convert the solar energy to
      chemical energy in a process  called photosynthesis.

3.   Gravity from the Sun keeps all of the planets (and our moon)  in their orbits. Earth remains in a nearly circular orbit, and our distance from the Sun insures that we receive just the right amount of heat to remain at a constant temperature (0°C-50°C). This is the only temperature range that can sustain life.

4.    The Sun is an average sized star, but is more than a million times greater in volume than the Earth.

5.    We can only see a few thousand stars from the earth with the naked eye, but it is estimated
       there are so many stars that we can't count them all.

6.    Other stars have basic similarities to our sun, but are so far away, they look like points of light.
       Distances to stars are vast compared to objects in our solar system.

light year -- the distance light travels in one year (used to measure distances between stars)

7.   Because the light coming from stars besides those in our own Milky Way galaxy show a
      red shift in spectrum, it is believed the stars in our universe are moving apart.

8.   During the course of an evening, all stars besides the North Star appear to move in a circular
      or semi-circular pattern.   This is NOT because the stars are moving, but is
      because the Earth is rotating on its axis.
  

See time lapse photography of star trails at the right which indicate the Earth's rotation.

Most objects in our solar system have a regular and predictable motion.  These motions explain such phenomena
as a day, a year, phases of the moon, eclipses, tides, meteor showers, and comets.

Earth Motions

A.     even though you can’t feel it, the earth is moving
        1.     we are traveling through space at about 18 miles per second
        2.     we can’t see or feel this because everything is traveling at the same speed

B.     rotation --  an object spinning on its axis (an imaginary line through its center)
        1.     Earth is spinning like a top on its axis (an imaginary line connecting the N and S poles)
        2.     The daily motions of the Sun, the Moon, the Planets and the Stars across the sky
                 can all be explained by a rotating Earth
        3.     takes 24 hours to complete 1 rotation (day)
        4.     the sun shines on the earth as it rotates
                a.     the sun’s light can only shine on ½ of the earth at a time
                b.     Earth's rotation in relation to the Sun causes
                        the daily change from daylight to darkness.  It is daytime on the lighted side
                        – nighttime on the opposite side
                c.     The counterclockwise rotation of the Earth makes it appear that the Sun and Moon
                        rise along the eastern horizon and set along the western horizon
                d.     At noon in the continental U.S., the shadow cast by a vertical pole points in a
                        northerly direction
                e.     as Earth moves, the part that the sun shines on changes as does the time of day at each
                        location
                f.     This daily change occurs everywhere on the Earth except within the Arctic and Antarctic
                        Circles.
               g.      The rotation of the earth is chiefly responsible for ocean and other tides.

C.     revolution -- the movement of one object around another
        1.     the earth is also moving through space
        2.     it is traveling in an path (orbit) around the sun
        3.     it takes 365 ¼ days (year) to complete one revolution

D.     seasons
        1.     the earth is tilted 23 ½ ° causing seasons
        2.     depending on the earth’s position relative to the sun, the sun’s rays will strike the earth directly on
                 the equator (equinox) or closer to the poles (solstice)
        3.     the tilt of the earth plus the earth’s position compared to the sun causes the seasons
                a. summer solstice – June 21st (first say of summer) (Northern hemisphere tilted toward Sun)
                b. autumnal equinox – September 23rd (first day of autumn) (neither hemisphere is tilted toward Sun, which is shining directly on the equator)
                c. winter solstice – December 21st (first day of winter) (Southern hemisphere is tilted toward Sun and the Northern hemisphere away)
                d. vernal equinox – March 21st (first day of spring) (neither hemisphere is tilted toward Sun, which is shining directly on the equator)
 

**  Note that the seasons are reversed between the Northern and Southern hemispheres.

 

Because of the tilt of the Earth's axis, the rays of the Sun strike us more directly in the Summer, so more of the Sun's rays are focused on a small area.   As a result, we are hotter in the Summer and  colder in the Winter.  The rays strike us less directly in the Winter. The effects of this are shown in the graphic at the right.

The Winter low Sun angle with its rays spread out over a greater area (so they have less heating effect) is on the left.   The Summer graphic is on the right.

**  The length of daylight depends on latitude and season.

latitude -- the angular distance north or south of the equator

Lines of latitude called parallels, run east to west

longitude -- the angular distance east of west of the Prime Meridian (an imaginary line running from pole to pole through Greenwich, England)

Lines of longitude are called meridians.

Some Facts:

1.   The North Pole (90 N) and South Pole (90 S) has six months of constant daylight followed by
      six months of constant darkness.

2.   Our latitude (about 45 N) has about the following amounts of daylight and darkness depending on time
      of the year.
 

Time of Year	Hours of Daylight	Hours of Darkness
Vernal equinox  3/21	12 hours	12 hours
Summer solstice 6/21	15 hours	9 hours
Autumnal equinox  9/23	12 hours	12 hours
Winter solstice  12/21	9 hours	15 hours

3.   The equator has approximately equal lengths of day and night all year long every day.
 

Moon Motions and Information

The Moon orbits the Earth roughly once a month.

Looking down on the Earth and Moon from above the Earth's north pole, we see that its revolution is in the same direction as the Earth's rotation (and also the Earth's revolution around the Sun).

The Moon shines by reflected sunlight.

Therefore, at any time only one half of the Moon, the side facing the Sun, is illuminated or lit up..

The illuminated side of the Moon is not necessarily the half which faces the Earth.

Depending on the relative positions of the Sun, Moon, and Earth, we see different fractions of the Moon illuminated.

These are called the phases of the Moon.
 

Some Facts relating to these phases:

At new moon, we can't see any of the illuminated half of the Moon; at full moon we can see all of it.

Halfway in between new and full moon, we see half of the illuminated half of the Moon, or a quarter of the Moon.

First quarter occurs as the Moon moves from new to full; third or last quarter occurs as the Moon moves from full to new.

Between the new and quarter moons, only a small fraction of the Moon is illuminated; we call this a crescent moon.

Between the quarter and full moons a larger fraction of the Moon is illuminated; we call this a gibbous moon.

On any particular night, the Moon will seem essentially motionless.

As can be seen from the diagram above, a full moon must therefore rise around 6 P.M., be overhead at midnight, and set around 6 A.M.

A first quarter moon must rise around noon, be overhead around 6 P.M., and set around midnight.

Crescent moons are overhead during the day, but they are generally only visible near sunrise/sunset (both because of their small illumination and the brighter light from the Sun).

Eclipses -- occur when one celestial (space) object blocks the light from another

lunar eclipse -- occurs when the earth's shadow blocks the light of the moon from reaching the earth

solar eclipse -- occurs when the moon blocks the light from the sun from reaching part of the earth

For more information on lunar phases and eclipses, please click on the web link from Emory University
where the images above were posted.

Tides:  occur twice daily and are caused by the pull of the Moon (mostly) and the Sun (to a lesser extent .. 48% of
            moon effect)

spring tide -- highest tide which occurs when the sun and moon pull on a place on the earth together

neap tide -- lowest tide when the sun and moon pull at right angles to each other
 
 

Link to an animated view of spring and neap tides
 

Solar System Models

1.  geocentric model -- ancient idea the earth was the center of the universe

2.  heliocentric model -- the current correct sun centered solar system model

Our Solar System

The order of the planets in increasing distance from the Sun is as follows.

1.    Mercury
2.    Venus
3.    Earth
4.    Mars

**  Most of the asteroids (tiny planets which orbit the sun) are located in the area between Mars and Jupiter.

5.    Jupiter
6.    Saturn
7.    Uranus
8.    Neptune
9.    Pluto

A link to more information on the planets
 

While most of the planets have orbits which are circular or nearly circular around the Sun, comets have very oval or elliptical orbits. The orbit of a comet still follows a regular path and time however ... like the famous Halley's comet which returns approximately every 76 years to our view.

Information on Comet Composition
 
 

Note that the shape of the planets is nearly spherical.   The pictures at the right of our planet Earth and the Earth and Moon illustrate this.

 

meteor -- is cause by a rock fragment traveling in space that enters and heats in the Earth's
                atmosphere, producing light.