Earth, Sun, And Moon System

Earth, Sun, and Moon SystemIn the image on the right, you see Earth in the foreground,our tiny Moon, and our Sun in the distance. The Moonorbits Earth, and the Earth-Moon system orbits the Sun. Itis hard to imagine that the interactions of these threecelestial objects, although so far away from each other,can cause day and night, seasons, Moon phases,eclipses, and tides.Understanding the size comparison of the Sun,Earth, and Moon will help you understand theirmotion. Earth is much bigger than the Moon.About 50 Moons could fit inside Earth. Tocompare diameters, you could line up fourMoons across Earth.The Sun is even larger. About one millionEarths could fit inside the Sun, and 100 Earthscould fit across the face of the Sun.Earth, Moon, and Sun system motionsRotation is the motion of a spinning object. TheSun and each of the planets and moons in oursolar system rotate about an axis. An axis is animaginary line about which each planet or moonspins. This imaginary line marks the center of aplanet’s or moon’s rotation. Earth rotates on itsaxis about once every 24 hours, while the Moonrotates very slowly over the course of a month.Revolution is orbiting another body. Earth (withour Moon) revolves around the Sun in one year(365¼ days). The Moon revolves around Earthin one month (approx 27.3 days). Because theMoon rotates once and revolves once a month,the same side always faces Earth.Not to scale!1

Earth, Sun, and Moon SystemEarth does not tilt back and forth as it goes around the Sun. Earth revolves at a constant angle of23.5 around the Sun. The northern end of Earth’s axis, the geographic north pole, always pointsto the North Star, Polaris.Day and NightWhat do you think causes day and night onEarth? Rotation! As Earth spins on its axis, theside facing the Sun experiences daytime(about 12 hours), while the side of Earth facingaway from the Sun experiences night (about 12hours). The same occurs on the Moon, exceptthe Moon’s slow rotation causes daytime to lastabout two weeks and night to last another twoweeks. So a full day on the Moon would lastabout one month.Even though you can’t feel it, Earth rotates very fast. Earth’srotation is so fast that it causes the planet to bulge out slightly atthe equator and shrink slightly at the poles.Therefore, Earth is not a perfect sphere. Earth’s circumference isslightly wider at the equator than it is across the poles. This shapeis called an oblate spheroid. In most photographs and diagrams,Earth typically looks like a perfect sphere. Because of this shape,solar energy travels in a straight line from the Sun and hitsdifferent parts of the curved Earth at different angles—moredirectly at the equator and less directly at the poles.This clementine is anoblate spheroid. Earthis more rounded thanthis clementine.2

Earth, Sun, and Moon SystemTo make a good model of the Earth, Sun, andMoon System, you need to choose objects thateither show size and location or objects useful forshowing their movements. Now you need toconsider distance. Even though your distances willnot be exact, your model will still help show thepatterns of how these objects move. The Moonrevolves around Earth at a distance of about 30Earth diameters. The Moon is about one--quarter the diameter of Earth. Knowing those twodimensions should help you create a scale model inside the classroom. In the model above, theyellow balloon is the Sun, the ping-pong ball is Earth sitting on a golf tee poking through the holein the ruler, and the mini-marshmallow is the Moon attached to Earth with a short straw.Although it appears to be glowing, the Moon does not produce its own light. Instead, the face of theMoon is illuminated by the Sun. Solar energy travels in a straight line from the Sun to Earth and theMoon so that the side of Earth or the Moon that faces the Sun is illuminated. Solar energy reflectsoff of the side of the Moon that faces the Sun and can travel to Earth. As it revolves around Earth,the Moon reflects different amounts of sunlight. As a result, we see different amounts of the Moonat different times of the month.The Lunar CycleOur view of the sunny side of the Moondepends on the relative positions of Earth, theMoon, and the Sun. This happens in apredictable pattern called the lunar cycle, inwhich we observe different phases of theMoon. This diagram shows the eight phasesof the lunar cycle. For example, in thediagram you can see that the full Moon phaseoccurs when the Moon, Earth, and the Sunare in a straight line, resulting in the full Moonappearing all night. However, when the Mooncomes between Earth and the Sun, as in thenew Moon phase, the Moon rises and setsduring the day and is not visible. Each phaseis described in greater detail on the nextpage.The lunar cycle occurs because the Sunilluminates varying amounts of the Moon facingEarth observers.3

Earth, Sun, and Moon SystemThe Phases of the MoonNew Moon occurs when the Moon is located directly between Earthand the Sun. As a result of this alignment, the Sun illuminates the sideof the Moon that faces away from Earth. The Moon is not visible fromEarth at night during the new Moon phase because it rises and setsduring the day.Waxing crescent occurs when the new Moon begins to change to afirst-quarter Moon. (Waxing describes the period when we see moreand more of the Moon from Earth.) During this phase, the Sunilluminates less than half of the side of the Moon facing Earth. As aresult, the Moon appears as a crescent shape in the sky.First quarter occurs when the waxing Moon is at a 90 angle relativeto Earth and the Sun. During the first-quarter phase, the Sunilluminates exactly half of the side of the Moon facing Earth. Thefirst-quarter phase is also called a half Moon.Waxing gibbous occurs as the first-quarter Moon changes to a fullMoon. During the waxing gibbous phase, the Sun illuminates morethan half of the side of the Moon facing Earth. As a result, the Moonappears nearly full in the sky.Full Moon occurs when Earth is located between the Moon and theSun. During the full Moon phase, the Sun illuminates the whole sideof the Moon facing Earth. During this phase, the Moon appears as afull circle in the sky.Waning gibbous occurs as the full Moon changes to a last-quarterMoon. (Waning describes the period when we see less and less of theMoon from Earth.) The waning gibbous phase is the mirror image ofthe waxing gibbous phase.Last (third) quarter occurs when the waning Moon is at a 90 anglerelative to Earth and the Sun. The last-quarter phase (also called ahalf Moon) is the mirror image of the first-quarter phase.Waning crescent occurs when the last-quarter Moon changes to anew Moon. The waning crescent phase is the mirror image of thewaxing crescent phase. A new Moon follows the waning crescentphase, starting the cycle over again. The Moon takes approximately28 days to complete one cycle.4

Earth, Sun, and Moon SystemThe fact that Earth’s distance from the Sun changesthroughout the year might seem like a good explanation for theseasons. You might think that Earth is colder when it is fartherfrom the Sun and warmer when it is closer to the Sun;however, this explanation is incorrect. In fact, the distancebetween Earth and the Sun stays relatively constantthroughout the Earth’s orbit. Also, not every part of Earthexperiences the same seasons at the same time. When it issummer in the northern hemisphere, it is winter in the southernhemisphere. What, then, causes the seasons?SeasonsEarth’s axis is tilted at a 23.5 angle. If Earth’s axis were not tilted, we would not experiencedifferent seasons. As Earth revolves around the Sun, sometimes the northern hemisphere is tiltedtoward the Sun. When the northern hemisphere is tilted toward the Sun, it receives more directrays of sunlight. It is summer. During summer, days are longer and weather is warmer in thenorthern hemisphere. Plants there have plenty of sunlight for photosynthesis, and animals haveplenty to eat.Earth’s tilted axis and its revolutionaround the Sun—not Earth’s distancefrom the Sun—cause the seasons.When the northern hemisphere is tiltedtoward the Sun, it receives more directrays of sunlight than the southernhemisphere. During this time, thenorthern hemisphere experiencessummer and the southern hemisphereexperiences winter.When Earth reaches the opposite side of its orbit—a process that takes about six months—thesouthern hemisphere is tilted toward the Sun. It will receive more direct and intense solar energy,and the northern hemisphere will receive less. As a result, the southern hemisphere willexperience summer and the northern hemisphere is in the middle of winter. The change in seasonat a given place on Earth is directly related to the orientation of the tilted Earth and the position ofEarth in its orbit around the Sun because of the change in the directness and intensity of the solarenergy at that place over the course of the year.5

Earth, Sun, and Moon SystemEclipsesHow do you think the Earth, Sun, and Moon System can explain eclipses of the Sun and theMoon? An eclipse occurs when one celestial object passes through the shadow or is put intoshadow by another celestial object. Solar Eclipse: When the Moon blocks the Sun and the Moon’s shadow falls along a small pathon EarthLunar Eclipse: When the Moon moves into Earth’s shadow.We do not see solar and lunar eclipses everymonth because the Moon’s orbital path aroundEarth is tilted with respect to the plane of theEarth’s orbit. Eclipses only occur when all threecelestial bodies line up in the same plane. Thepoints in the Moon’s orbit where this occurs arecalled nodes (shown on the left as red dots).Eclipses only occur at these nodes.Solar energy is preventedfrom reaching Earth during asolar eclipse because theMoon is located between theSun and Earth.Solar energy is preventedfrom reaching the Moon (andthus reflecting off of theMoon to Earth) during alunar eclipse because Earthis located between the Sunand the Moon.Because the Moon’s orbitalplane is tilted with respect tothe plane of Earth’s orbitaround the Sun, for a majority of time during an Earth month, the Moon is not in a position to blocksolar energy from reaching Earth, and Earth is not in a position to block solar energy from reachingthe Moon.6

Earth, Sun, and Moon SystemEarth’s rotation and revolution affect day/night cycles as well as the seasons. The followingdiagram shows Earth at two positions in its orbit around the Sun. (This diagram is not drawn toscale.)For each position, decide whether each hemisphere is experiencing day or night and winter orsummer. Write your answers in the charts below.Position 1NE HemisphereNW HemisphereDay orWinter orDay orWinter orNight?Summer?Night?Summer?Position 2NE HemisphereNW HemisphereDay orWinter orDay orWinter orNight?Summer?Night?Summer?Position 1SE HemisphereSW HemisphereDay orWinter orDay orWinter orNight?Summer?Night?Summer?Position 2SE HemisphereSW HemisphereDay orWinter orDay orWinter orNight?Summer?Night?Summer?7

Earth, Sun, and Moon System Understanding the size comparison of the Sun, Earth, and Moon will help you understand their motion. Earth is much bigger than the Moon. About 50 Moons could fit inside Earth. To compare diameters, you could line up four Moons across Earth. The Sun is even larger. About one million