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Welcome to PH109Exploring the Universe

Dr. Michael L. CobbFall, 2003

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• Pass out Syllabus

• Web Page

• Night Lab

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Goals

• Describe Celestial Sphere

• Locate Objects in the Sky

• Explain How We Measure Distances

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The Scale of Things

Units change for different scale lengths.

Km for close objects.

Light Years for Distant objects.

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The Universe is so vast that it contains scale sizes far larger than our ordinary, everyday experiences.

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Constellations

Constellations provide 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.

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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

Not very useful for the stars since it depends on the time and the position of the observer

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Home Work #1

Locate the planet Venus in the western sky and determine its Azimuth and Elevation.

Turn in the answer next lecture.

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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.

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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.

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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.

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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.

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The Zodiac

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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

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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.

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While the Moon orbits the Earth about once per month, half of it is always illuminated by the Sun and half is in its own shadow.

Because of the Sun-Earth-Moon angle the fraction of the lit part that can be seen at any one time changes and we call this the phases of the Moon.

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If the plane of the Moon’s orbit were level with respect to the Earth’s orbit, we would get a lunar eclipse every full moon and a solar eclipse every new moon.

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But the Moon’s orbit is tipped and so most of the time the moon misses the Earth’s shadow and the Earth misses the Moon’s shadow.

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Next Solar eclipse in next year in Europe

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The Earth and Moon do line up about twice per year and when they do we get eclipses.

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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

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Try it.

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If you know the size of an object then its angular size is related to its distance from us. If its distance doubles then its angular size is reduced by 1/2.

Thus there is an inverse relationship between angular size and distance.

Astronomers can determine distances to objects by assuming their size and measuring their angular size.