what time is it? depends on where you are on the earth! time zones ensure that the noon is really...

What time is it?

• Depends on where you are on the Earth!• Time zones ensure that the noon is really

noon, i.e. sun is at highest point• To avoid confusion, use universal time (UT),

the time at the meridian in GreenwichUT = EST + 5 hrs

• Daylight savings adds one hour in spring, so UT = EDT+ 4 hrs

The Time Zones

Established to insure that sun is at highest point approximately at noon in the middle of the time zone

Daily and yearly motion intertwined

Solar vs Siderial Day – Earth rotates in 23h56m

– also rotates around sun

needs 4 min. to “catch up”

Consequence: stars rise 4 minutes earlier each night (or two hours per month, or 12 hours in ½ year)

After 1/2 year we see a completely different sky at night!

Carl Sagan Article: Baloney Detection Kit

• Occam's Razor

• Authorities do NOT carry a lot of weight

• Ask whether the hypothesis can be falsified

• Use MANY hypotheses to explain experimental facts

• There must be an independent confirmation of the facts

• Quantify!

Bacon: The subtlety of Nature is greater than the subtlety of

argument.

• Often Nature is much weirder than we think (or are used to from our every day experiences)– At very large speeds (Relativity)– For very small objects (Quantum Mechanics)– For very dense objects (Black Holes)– Etc.

Fallacies of logic and rhetoric

• Ad hominem

• Non sequitur

• Appeal to ignorance

• Begging the question

• Observational selection

You may influence public opinion, but for scientific progress, all that matters is agreement with observations

Seasonal Motion

• Daily Rising and Setting:– Due to the rotation of the

Earth around its axis– Period of rotation: 1

siderial day= 23h56m4.1s – 1 solar day (Noon to Noon) =24h

– Stars rotate around the North Star – Polaris

• Seasonal Changes:– Monthly differences caused

by Earth’s orbit around sun

The Zodiac throughout the Year

Example: In Winter sun in Sagittarius, Gemini at night sky; in summer sun in Gemini, Sagittarius at night sky

Another Complication: Axis Tilt!• The Earth’s rotation axis is tilted 23½ degrees

with respect to the plane of its orbit around the sun (the ecliptic)

• It is fixed in space sometimes we look “down” onto the ecliptic, sometimes “up” to it

Path around sun

Rotation axis

Activity: The Ecliptic

• Get out your activities book

• Form a group of 3-4 people

• Work on the questions

• Hand in a sheet of paper with the title of the activity and the names of the group members

• I’ll come around to help out !

The Seasons• Change of seasons

is a result of the tilt of the Earth’s rotation axis with respect to the plane of the ecliptic

• Sun, moon, planets run along the ecliptic

Animation

• TeacherTube video

Position of Ecliptic on the Celestial Sphere• Earth axis is tilted w.r.t. ecliptic by 23 ½ degrees

• Equivalent: ecliptic is tilted by 23 ½ degrees w.r.t. equator! Sun appears to be sometime above (e.g. summer

solstice), sometimes below, and sometimes on the celestial equator

Zodiacal signs vs. Constellations

- 360/12=30, so each zodiacal sign is exactly 30 degrees “long”- 0 degrees: Aries, 30 degrees: Taurus, 60 degrees: Gemini, 90

degrees: Cancer, etc.

•“Constellation” is a modern, well-defined term

- Some constellations are big, some are small on the celestial sphere

•“Zodiacal sign” is the old way of dividing the year and the Sun’s path into 12 equal parts

Example

The vernal equinox happens when the sun enters the zodiacal sign of Aries, but is actually located in the constellation of Pisces.