Published on March 5, 2014
The View from Earth Chapter 12 Lesson 1 p414-420
Vocabulary • Spectroscope (417) – an instrument that spreads light into different wavelengths • Astronomical Unit (418) – the average distance between Earth and the Sun, about 150 million km • Light-year (418) – the distance light travels in 1 year • Apparent Magnitude (419) – a measure of how bright it an object appears from Earth • Luminosity (419) – the true brightness of an object
Looking at the Night Sky • If you look at the stars for a long time they seem to move. • Why does this happen? • Polaris is a star almost directly above the north pole. • As Earth spins Polaris stays in place as stars near it seem to circle around it.
• Polaris is commonly referred to as the Northern Star, because it and the stars near it never set when viewed from the northern hemisphere.
Naked-Eye Astronomy • Naked-eye astronomy means gazing at the sky using just your eyes; no binoculars or telescopes. • Before telescopes, people used the stars to tell time, seasons, and finding directions.
Constellations • When ancient cultures gazed at the night sky, they saw patterns. – They represented people, animals, and objects • The Greek astronomer Ptolemy identified dozens of star patterns nearly 2000 years ago. – These are called ancient constellations today • Present-day astronomers use many ancient constellations to divide the sky into 88 regions also called constellations
Telescopes • Telescopes are able to collect much more light than the human eye • The electromagnetic spectrum is a continuous range of wavelengths – Visible light is only one part of the spectrum – Longer wavelengths have low energy – Shorter wavelengths have higher energy • Different objects in space can emit different types of wavelengths. • The range of wavelengths a star emits is called its spectrum.
Spectroscopes • Scientists study the spectra of a star using a spectroscope. • A spectroscope spreads light into different wavelengths. • This way scientists can study a stars’ characteristics, compositions, and energies. – Newly formed stars emit mostly radio and infrared waves. While exploding stars emit high energy ultraviolet waves.
Measuring distance • Astronomers use angles created by parallax to measure how far objects are from Earth. – Parallax is the apparent change in an object’s position caused by looking at it from 2 different points. • For example: Look at your pencil with only your left eye. Then, without moving the pencil, look at it with only your right eye. • Astronomers create a parallax by using 2 points in Earth’s orbit around the Sun
Distance Within the Solar System • Distances within the solar system are measured using astronomical units or AU. – An astronomical unit is the average distance between Earth and the Sun, about 150 million km.
Distances Beyond the Solar System • Astronomers measure distances beyond the solar system using light-years. – A light-year is the distance light travels in 1 year. – 1 light-year equals about 10 trillion km. – The nearest star to our Sun is 4.2 light-years away. How far is that in km?
Looking Back in Time • Because it takes time for light to travel, you see a star not as it is today, but as it was when light left it. • At 4.2 light-years away, Proxima Centauri appears as it was 4.2 years ago.
Measuring Brightness • Astronomers measure the brightness of stars in two ways: – By how bright they are from Earth – By how bright they actually are
Apparent Magnitude • Scientists measure how bright stars appear from Earth using a scale developed by the ancient Greek astronomer Hipparchus – Hipparchus assigned a number to every star he saw in the night sky based on its brightness. • Today these are called number magnitudes • The apparent magnitude of an object is a measure of how bright it appears from Earth – Hipparchus assigned the number 1 to the brightest star he saw in the night sky
Absolute Magnitude • Stars can appear bright or dim depending on their distances from Earth, but stars also have actual, or absolute, magnitudes • Luminosity is the true brightness of an object • This depends on the star’s temperature and size rather then its distance from Earth • A star’s luminosity, distance, and apparent magnitude are all related. Thus, if a scientist knows two of these factor, he can mathematically determine the third.
To view the Earth as currently seen from a satellite in Earth orbit, choose the satellite from the list below and press the "View Earth from Satellite" button.
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