Time & Earth’s Motions - Mr. Goodyear...

Objectives: s.w.b.a.t. Explain how time zones where

established

Define: rotation – siderial vs synodic and the difference between each

Explain how we know the Earth rotates

Explain what a time zone and how it is defined

Explain solar mean time vs sundial time

Activities:

Warm-ups

Review cross-section problems – as

sign more for H.W.

Notes/discussion Time zones/Earth motions

Time & Earth’s Motions

Rotation – spinning of the Earth on its axis = 24 hrs.

causes apparent motion of objects in the sky to move E to W

True motion is from west to east

15º/hr or 1º/4 min.

Proof that Earth’s rotates

1. 1852 experiment jean Foucault (pendulum changes direction)

2. Corialis effect – apparent curved path of wind, missiles, planes. (north things veer to right, south veer to the left)

3. Shape of the Earth (oblate spheroid – bulge at the equator)

4. Space flight

Two ways or references of rotation:

Synodic rotation – 24 hours based on the sun lining up on the celestial meridian

Sidereal rotation – 23 hours 56 minutes based on a star lining up on the celestial meridian

Why is there a difference?

Because the Earth is revolving around the sun as it is rotating

360 º in one rotation take 24 hrs. 360/24 = 15 60/15 = 4 min. Or 1º/4 min.

360º in one revolution(orbit around the sun) 360/365 ~1º a day change on its orbital motion.

So due to this the Earth must rotate an extra 4 min. to line back up with the sun so one can see from this motion:

*** a given star rises for min. earlier each day. So if a star A rises at 9:00 pm today next day will rise at 8:56pm

Reference pages : __________

Because the sun is located at different altitudes and positions, places on Earth (longitude concern) different places have different times.

Start at the Prime meridian 0º longitude in Greenwich England (universal time)

Local apparent time: time kept by a sundial: depends on season and location on Earth

Local mean time: (watch) – is the average of the local apparent time for a given time zone or longitude belt.

Sundial time(local apparent time) and the local mean time(watch) are usually not the same. The difference between the two is called Equation of Time.

It is never more than + or – 17 min. during the year, a person simply adds or subtracts for the current date to correct sundial time to local mean time (see Analemma for correction)

-above does not take into account daylight savings time changes

Sundial problem: City A is 10º east of City B, it is 3:00pm at

city B, What time is it at City A?

Other things to keep in mind about time!!! As one goes East time gets later, as one

goes west time gets earlier What state has the longest day? shortest? As you go north or south from the Equator

the days get longer in summer and shorter in the winter months

the height of the sun’s vertical rays get lower as you go away from the equator.

Cross section problems

continue:

Working backward on cross section problems examples:

Your latitude is 31º S, What would the height of the sun be on June 21?

Your latitude is 62.5º N, What would the height of the sun be on Dec. 21?

Your latitude is 50º N,

What would the height of the sun be on June. 21?

What would be the height of your celestial equator?

What would the height of the sun on Sept.21?