welcome to ph109 exploring the universe dr. michael l. cobb fall, 2013

Welcome to PH109Exploring the Universe

Dr. Michael L. CobbFall, 2013

Charting the Heavens

• Pass out Syllabus

Web Page

• Night Lab – We currently have no observatory with night lab capability.

• Students must purchase a lab manual from the Bookstore (<$4).


Goals

• Describe celestial sphere

• Locate and explain motion of objects in the sky

• Explain why we have seasonal changes

• Explain the changing appearance of the Moon

• Explain the scientific method


The Scale of Things

Units change for different scale lengths.

Km for close objects.

Light Years for Distant objects.

The Universe is so vast that it contains scale sizes far larger than our ordinary, everyday experiences and encompasses about 1040 in scale .

Powers of Ten


Constellations

Constellations provide an easy way to locate objects in the sky.

Invented by ancient astrologers, constellations are used today has a historical basis. Modern astronomers place no significance on actual constellations.

Stars which appear connected are actually dispersed in three dimensional space.


Horizon Coordinate System

There are many kinds of coordinate systems. Street and number for houses, section, row, and seat for stadium. Using the correct system can simplify the problem. The simplest coordinate system is the Horizon system

•Azimuth - Degrees (0 - 360) from North through the East•Elevation - Degrees (0 - 90) above horizon•Meridian - Line which divides sky into East and West halves•Zenith - Point directly overhead


Home Work #1

Locate the bright planet Venus low in the west southwest sky after sunset and determine its Azimuth and Elevation.

Turn in the answer next lecture.


The Horizon coordinate system is not very useful as it requires observers to be in the same location at the same time.

Thus for your AZ/EL information to be useful you need to indicate the date, time, and place of observation.

We need a better coordinate system that is fixed to the stars.


Ancient astronomers thought the stars were fixed on a firmament above the sky.

Modern astronomers still use the concept of a celestial sphere where we “imagine” stars are fixed on a sphere even though we know this is not true.

While it is really the Earth that rotates, we imagine the sphere rotates about the Earth.


As the Earth rotates on its axis, it appears to us that the stars are rotating about a point called the North Celestial Pole (NCP).

The “North Star” happens to be near this NCP and so always appears to be in the North. Otherwise it has no special properties.

The Southern hemisphere has no “South Star” because there is no star close to the SCP.

The height of the NCP above the horizon is equal to your latitude.


To locate objects on Earth we use the coordinates of latitude and longitude. Latitude measures the angular distance from the equator. Longitude measures the angular distance from Greenwich England.

In astronomy we use the Equatorial coordinate system which uses Declination to measure distances from the celestial equator and Right Ascension to measure distances from the vernal equinox.


The Seasons

The Earth orbits around (revolves) the Sun in about 365.25 days (rotations).

Because of the Earth’s rotation the Sun, Moon, and Stars appear to rise in the East and set in the West in a daily (diurnal) motion.

Because of the Earth’s revolution about the Sun. The Sun’s position appears to drift across the stars. The path the Sun takes is called the ecliptic or Zodiac.

The ancient astronomers divided the Zodiac into 12 equal pieces (months) which we now call the signs of the Zodiac.


Our life has always depended on the stars. Seafarers needed to navigate and farmers needed to know when to plant.

Our modern calendar was based on the changing heavens.

We have seven days of the week because there were seven movable objects in the heavens.

We have 12 months in a year because there are about 12 lunar cycles in a year.

We have 360 degrees in a circle because it takes about 360 days for the Sun to return to the same spot among the stars.


The Zodiac


Because the Earth’s axis is tipped 23.5o with respect to its orbital plane (ecliptic). The Sun appears to follow a north and south motion across the equator in the course of a year which causes the seasons.

Winter solstice - southern mostSummer solstice - northern mostVernal equinox - springAutumnal equinox - fall

solstice = means sun to standequinox = means equal night


Not only is the Earth’s axis tipped with respect to its orbital plane, but the direction of the tip is also changing and makes a complete circle every 26,000 years.

While Polaris is now the North Star, in 12,000 year Vega will be our North Star.

If the Earth were tipped more than 23.5o on its axis the seasons would be more drastic. While if the Earth were not tipped at all we would have no seasons.


The Moon is always half lit and half in shadow (except during a Lunar eclipse as discussed below). The phases of the Moon that we see are caused because the Earth-Moon-Sun angle means we see different amounts of the lit and shadowed parts.


During a Lunar eclipse the Earth’s shadow fall on the Moon.

During a Solar eclipse the Moon’s shadow falls on the Earth.


The reason why we don’t get an eclipse at every full and new Moon is because the Moon’s orbit is tipped with respect to the Earth’s orbit and most of the time the shadows miss.


Future predicted Solar eclipses.


We get to see the great Solar eclipse of August 21, 2017


People get distance clues from the angles their eyes make. You get cross-eyed if you look at something very close.

Astronomers use a similar technique called parallax.

You can demonstrate parallax by first viewing your finger from one eye and then the other and seeing how its position shifts against the background stars.

Parallax


Try it.


Science, unlike politics, defines “truth” in a verifiable may and arrives at the correct solution by using the “scientific method”.

The scientific methods usually starts with an observation followed by a theory which make predictions for more observations. When the logic circle is complete and verifiable by others, we declare a truth (or law).


The Earth is Round!

Scientists knew the Earth was round at least by 200 B.C. by 1% error!

welcome to ph109 exploring the universe dr. michael l. cobb fall, 2013

Documents