Day and Night

April 24, 2012

Rotation When a body turns or spins in a

circle, this motion is ROTATION. The imaginary shaft on which it turns

is called an AXIS Earth rotates counterclockwise on an

axis that runs through it from the North Pole to the South Pole.

Day The period of direct

illumination from the sun is called DAY.

The period of time when we cannot see the sun because it is on the other side of the planet is called NIGHT.

Day and Night is the product of a spinning earth (traveling at a velocity of 28 km/min)

When we shine light on a globe, no matter where we place the globe, half the Earth is illuminated and half dark.

Always exactly half and half.

As you rotate, the earth is experiencing sunset and sunrise.

Big Ideas to Understand! In the presence of a luminous source

A sphere will always be half in the light and half in the dark

The half of the sphere in the light changes as the sphere rotates

Day and Night can be explained simply as a rotational phenomenon.

Not all days are created equal

Summer days are longer than winter In the extreme latitudes (N/S Pole), days

are endless for days on end. This is because of the Earth’s axis

(23.5˚) If we were absolutely perpendicular,

days would be 12 hours of light, followed by 12 hours of darkness.The longest day: June 21The shortest day: December 21Equal Days: March 21, September 21

How do we know!? Is there anyway we could use a lamp to test

our ideas of day and night. Stand so that your observer is experiencing

day, then night.Stand so that your observer is experiencing

Noon.Stand so that your observer is experiencing

midnightStand so that your observer is experiencing

sunriseStand so that your observer is experiencing

sunset

Timekeeping

Timekeeping The origin of the hour as the

subdivision of the day is obscure. We have a period of time divided into

two halves, each in four quarters. Then into hours as well!

Reliable start times. Noon:

Point half way between sunrise and sunset The moment when the Sun is at its highest point

in the sky, is the time you would set for 12:00The time when the Sun reaches its highest point

Local Noon: time assigned zero Also known as the time halfway between sunrise

and sunsetTime is accounted from this time.

Reliable Start Times: Midnight:

12 hours from noon.Local Midnight:

Also known as the time halfway between sunset and sunrise

Time Standard Time

We count the passage of time in two pairs of 12 hours. One starting at midnight The other at noon.

Military TimeTo avoid redundant numbers, time is

accounted from midnight in one 24 hour sequence

Either way, noon is 12:00

Time around the world. When Earth began to shrink due to

the expansion of the railroad, scheduling became a nightmare.

We needed a way to travel East/West. We needed a global timekeeping system.

Sir Sanford Fleming Sir Sanford Fleming (Canadian) devised a

plan for standard Worldwide timekeeping 1884 a delegation of international

timekeepers met in DC and adopted a system of 24 time zones Each with 15˚of longitudeStarting at zero on the line through

Greenwich EnglandAlways in whole hour increments.

Every clock was set depending on the zone and is still used today.

Evidence that the Earth is round, came from careful observations of lunar eclipses.

The shadows crossing the Moon ALWAYS present a curved leading edge and a cured trailing edge, indicating that the round object is casting the shadow.

Shadows

Read about Eratosthenes

(Page 73 in TE)

You are going to make similar shadow observations to

Erathosthenes, 2000 years ago.

Review

Longitude - - the lines which run from the North pole to the South Pole (top to bottom)

Latitude - - run around the globe (like ladder rungs). The half way line is called the equator.

Light Rays always travel in straight lines

Because the sun is so large and so far away, all rays of light hitting anywhere on Earth are parallel to one another.

A pole casts a shadow only when the sun shines on its side, not when the Sun shines right on its top.

Poles in a row on a flat surface cast identical shadows, poles on a curved surface cast shadows of different lengths.

The different lengths of shadows observed by Eratosthenes suggested to him that the surface if Earth was curved, not flat.