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Chapter 27: The Sun-Earth-Moon System

Ch. 27.1 Tools of Astronomy


Distant bodies emit radiation called electromagnetic radiation which includes: visible light, infrared and ultraviolet radiation, radio waves, microwaves, X-rays, and gamma rays.
Electromagnetic spectrum- all types of electromagnetic radiation arranged according to wavelength and frequency.

Wavelength and Frequency

EM radiation is classified by wavelength; the distance between peaks on a wave.
EM radiation also classified according to frequency; the number of waves or oscillations that pass a given point per second.
All types of EM radiation travels at the speed of light in a vacuum.
Astronomers choose their tools based on the type of radiation they wish to study.


Objects in space emit radiation in all portions of the EM spectrum.
A telescope collects more EM radiation and focuses it so that an image can be recorded. The diameter of a telescope's opening is the aperture; larger apertures collect more EM radiation.

Telescopes can collect light over long periods of time, making it possible to detect objects that are too faint for the human eye. Specialized equipment can be added to telescopes: spectrophotometer displays the wavelengths of radiation, photometer measures intensity of light.

Refracting and Reflecting Telescopes

Refracting telescopes- (refractors) used lenses to bring visible light to a focus; largest lens is called the objective lens.

Reflecting telescopes- (reflectors) uses mirrors to focus light.
Advantage of reflecting telescopes is that mirrors can be made larger than lenses, able to collect more light.

Most telescopes used for scientific study are located at high elevations where there is less atmosphere to blur images.

Telescopes Using Non-Visible Wavelengths

Infrared and ultraviolet radiation can be focused by mirrors.
X-rays cannot be focused by normal mirrors, so special designs are used.
Gamma rays cannot be focused, so telescopes determine the direction from which rays come from only.
Radio waves are collected with a large dish to collect the longer wavelengths. The dish plays the role of mirrors by reflecting waves to a point where the receiver is located. The receiver converts the radio waves to electric signals which can be stored in a computer.

The data are converted into visual images by a computer. Interferometry- a technique that uses images from several telescopes to produce a single image.

Space-Based Astronomy

Space-based telescopes allow astronomers to study radiation that would be blurred by our atmosphere.

Hubble Space Telescope

Launched in 1990. Designed to obtain sharp visible-light images as well as infrared and ultraviolet wavelengths.

The James Webb Space Telescope is the next-generation telescope. Set to launch in 2018, will take images in infrared primarily.


Robotic probes can be sent to make close-up observations and sometimes land to collect information.

  • 2005- Cassini spacecraft arrived at Saturn to take a detailed look at the moons and rings.
  • 2006- New Horizons was launched for Pluto and beyond. It arrived in 2015 where it will orbit for a year and then go beyond.
  • 2012- Curiosity landed on Mars to assess whether or not environmental conditions are favorable to microbial life.

Human Spaceflight

Before humans can safely explore space, we must learn the effects of space; radiation and weightlessness. From 1981-2011 the space shuttle provided an environment to study the effects of weightlessness on humans, plants, the growth of crystals, and other phenomena.

The International Space Station allows for long-term studies in space.

Spinoff Technology

More than 1,500 different NASA technologies have been passed on to commercial industries for common use: memory foam, pumps for artificial hearts.

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Ch. 27.2 The Moon

The Moon is unique among the moons in our solar system.

Exploring the Moon

Much was learned by using telescopes but most knowledge of the Moon comes from explorations by space probes (Kaguya, Lunar Reconnaissance Orbiter). In 1957 Russia launched Sputnik I, the first artificial satellite. Four years later a Russian cosmonaut became the first human in space.

That same year, Alan Shepard, Jr. became the first American in space during Project Mercury. Project Gemini which launched two-man crews. July 20, 1969, Apollo program landed Neil Armstrong and "Buzz" Aldrin on the moon (Apollo 11). Using lunar roving vehicle, they explored several areas of the moon.

NASA hopes to return to the moon, even establish a base. Helping private companies to build piloted spacecraft.

The Lunar Surface

The lunar surface is dark. Albedo- the percentage of incoming sunlight that is reflected. Moon's albedo is very small, about 7%. Extreme temperature changes between the light and dark sides since no atmosphere (127ºC to -233ºC).

Surface features:

  • Highlands- heavily cratered, mountainous regions that are light in color.
  • Maria- dark, relatively smooth plains that are lower than the highlands.
  • Rilles- valley like structures that may be collapsed lava tubes.

Lunar Craters

  • Impact craters- craters on the moon that were formed when objects from space crashed into the lunar surface.
  • Ejecta- material blasted out during an impact that then falls back to the Moon's surface.
  • Rays- Long trails of ejecta that radiate outward from the impact site; visible as light-colored streaks.

Lunar Properties

Moon properties are unique among other moons.

  1. Largest moon to radius and mass of the planet it orbits.
  2. Is a solid, rocky body in contrast with the icy composition of most other moons.
  3. Orbit is farther from Earth relative to the distance of many moons from the planets they orbit.


    Made of minerals similar to those of Earth, mostly silicates. Highlands are predominantly lunar breccias, composed of plagioclase feldspar that formed by the fusion of smaller pieces of angular rock during impacts.

    Maria are mostly basalt. Unlike basalt on Earth, it contains no water.

History of the Moon

Radiometric dating of rocks from the highlands indicates an age between 3.8 and 4.6 billion years; about the same age as Earth. Scientists theorize that the Moon was heavily bombarded during the first 800 million years, causing a the breaking and heating of surface rocks. This formed the ground up rock called regolith on the surface.

Layered Structure

Using seismic data, scientists infer that the Moon has a layered structure similar to Earth.

  1. Crust: varies in thickness (thicker on far side).
  2. Upper mantle: is solid.
  3. Lower mantle: thought to be partially molten.
  4. Core: mostly solid iron.

Formation of Maria

Scientists estimate the maria formed between 3.1 and 3.8 mya. After bombardment that formed highlands, lava welled up from interior and filled the large impact basins. Flowing lava in the maria formed rilles, which are similar to lava tubes on Earth.

Virtually no maria on the far side of the Moon, which is covered almost completely with highlands. Scientists theorize this is due to thicker crust not allowing lava to the surface.


Scientists think that the Moon is not tectonically active. It has no volcanoes and no significant magnetic field but seismic data shows annual moonquake. Shapes and locations of mountains on the Moon indicate they were not created tectonically.


The most accepted theory today is the giant impact theory: a Mars-sized object collided with Earth about 4.5 billion years ago causing materials from the object and Earth's outer layers to be ejected into space. Gravity trapped the debris, which then began orbiting the Earth, merging together to form the Moon.

This theory has been accepted because of the similarities that have been found between bulk samples of rock taken from Earth and from the Moon.

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Ch. 27.3 The Sun-Earth-Moon System

Motions of the Sun-Earth-Moon system define Earth's day, month, and year.

Daily Motions

A result of Earth's rotation is that the Sun, moon, planets, and stars rise in the east and set in the west.

Earth's Rotation

Two ways to demonstrate that Earth is rotating:

  1. Foucault pendulum: pendulum swings in a constant direction; pendulum direction shifts when Earth rotates.
  2. Air is diverted from north-south direction to east-west direction as a result of the Coriolis effect.

Day Length

Solar day is measured as the time from one noon to the next; we base our time keeping system on the solar day.
Our observed day is about four minutes longer than solar day. This is a result of the advancement of Earth in it's orbit, so it has to rotate a little further to line up with the Sun.

Annual Motions

Ecliptic Plane- the plane of Earth's orbit. Earth orbit's the Sun in a slightly elliptical orbit. The path of the object in the sky is known as the ecliptic.

The Effects of Earth's Tilt

Earth is tilted 23.5º relative to the ecliptic. The seasons are a result of the Earth's tilt and orbit.


Solstice- when the sun is overhead at its farthest distance either north or south to the equator.

June 21, summer solstice, daylight hours reach the maximum for northern hemisphere. The Tropic of Cancer at 23.5º Norther latitude corresponds.
December 21, winter solstice, daylight hours reach the minimum for northern hemisphere. The Tropic of Capricorn at 23.5º South latitude corresponds.


Midway between solstices, represent the equinoxes (means equal nights). Equinox- Earth's axis is perpendicular to the Sun's rays, at noon the Sun is directly over the equator. Autumn equinox and Vernal equinox.

Changes in Altitude

The Sun's maximum height at midday is referred to as the zenith.

Phases of the Moon

Lunar phases is the sequential changes in the appearance of the Moon. One half of the Moon is reflecting the Sun's light at all times, how much we see depends on the position of the Moon as it revolves around Earth. New Moon is when the Moon is between the Earth and the Sun.

Waxing and Waning

The increase in the visible sunlit surface of the Moon is called the waxing phase which includes: crescent, first quarter, and gibbous. After the full moon, the sunlit side that is visible decreases; this is called the waning phase and includes: gibbous, third quarter, and crescent.

Synchronous Rotation

As the moon orbits Earth, the same side faces Earth at all times. This is because the Moon spins on its axis at the same rate of its orbit. Scientists theorize the Earth's gravity slowed the Moon's original spin until the Moon reached synchronous rotation. Synchronous rotation- the state at which its orbital and rotational periods are equal.

Lunar Motions

A lunar month- time it takes for the Moon to go though a complete cycle of phases. Lunar month length is about 29.5 days, which is longer than the 27.3 days it takes for one revolution.


The Moon's gravity pulls on Earth crating bulges of ocean water on both the near and far sides of Earth. Earth's rotation also contributes to the tides. As Earth rotates, the bulges remain aligned with the Moon. Ocean level rises and falls every 12 hours.

Spring and Neap Tides

The Sun's gravitational pull also affects tides, but half that of the Moon since it is farther away.

  • Spring- when Sun and Moon align along the same direction, gravitational effects combine and tides are higher than normal.
  • Neap- When the Moon is at a right angle to the Sun-Earth line, tides are lower than normal as a result of gravitational forces competing with each other.

Solar Eclipses

Solar eclipse- when the Moon passes directly between the Sun and Earth and blocks the Sun from view.
Total solar eclipse occurs when the Moon perfectly blocks the Sun's disk so that only the dim, outer gaseous layers are visible.

How Solar Eclipses Occur

Each object in the solar system creates a shadow as it blocks the path of the Sun's light. The shadow is totally dark directly behind the object and has a cone shape.

The Moon casts a shadow that consists of two regions: umbra and penumbra.

  • Umbra- the inner, cone-shaped portion that blocks the direct sunlight creating a total eclipse.
  • Penumbra- outer portion of the shadow where some of the Sun's light still reaches Earth creating a partial solar eclipse.

Umbral shadow is typically never wider than 270km.

Effects of Tilted Orbits

The Moon's orbit it tilted 5º relative to the ecliptic plane, as a result the Moon passes above or below the Sun as seen from Earth. A solar eclipse occurs only when the intersection of the Moon and the ecliptic plane is in a line with the Sun and Earth.

Annular Eclipses

In addition to tilt of Moon's orbit, the Moon's distance from Earth changes. Perigee- the closest point in the Moon's orbit to Earth. Apogee- the farthest point.

Earth's closest point to Sun is perihelion, farthest point is aphelion.

Annular eclipse is when a ring of the sun called the annulus appears around the dark Moon during an eclipse.

Lunar Eclipse

Lunar eclipse- when the moon passes behind Earth in relation to the Sun, and through Earth's shadow. Lunar eclipse only occurs during a full Moon.
Earth's shadow has umbra and penumbra. Light bent from atmosphere allows the Moon to be faintly visible and may create a reddish color.

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Page last updated January 2, 2017.