The Earth, Moon, And Sky - Physics & Astronomy
The Earth, Moon, and SkyLecture 51/31/2017
From Last Time:Stable Orbits The type of orbit depends on theinitial speed of the object Stable orbits are either circularor elliptical. Too slow and gravity pulls theobject back in Too fast and gravity is not strongenough. This is called the escapevelocity
From Last Time:Difference between Mass and Weight Newton’s Second Law: Force mass x acceleration Mass is a measure of how much material an object has. Weight (force) is a measure of home much acceleration a mass isfeeling. Earth’s surface acceleration 9.8 m/sec2 - My weight is 889 Newtons (mass 90.7 Kg) Jupiter’s surface acceleration 24.79 m/sec2 - My weight is 2248.4 Newtons We as a society conflate mass and weight since everyone feels thesame gravity. We mix up Newtons and Kilograms, though not thesame.
Locating Places on Earth We live on a sphere and thus define our positionusing two angles. Latitude measures North/South angular distanceswith 0 degrees at the Equator Longitude measures East/West angular distanceswith 0 degrees on the Prime Meridian(Greenwich, England). Washington, DC is at 38.9072 N, 77.0369 W Albuquerque is at 35.0853 N, 106.6056 W Melbourne Australia is at 37.8136 S, 144.9631 E
The Seasons The Earth is on an elliptical orbit, sometimes it is farther away to Sun,sometimes it is closer. Does this create Seasons? It is summer in the southern hemisphere. Earth is actually its closest to the Sun in January. Earth varies its distance to the Sun by about 3% over a year. Conclusion: Earth changing its distance to the Sun create seasons. Axial tilt, 23.5o, does create seasons. The Suns position in the sky changes throughout the year because ofthe tilt
Axial Tilt:o23.5 Summer occurs for a certain hemisphere, when it is “leaning”towards the Sun. Hemispheres distance to the Sun is changing, but it is insignificantrelative to the distance from the Sun to the Earth. Sun light is more direct in Summer and more spread out in Winter.SummerWinter
Seasons also depend of the amount of daylight.
Solstices Around June 21, the Sun is themost over head it will be all year. At Latitude 23.50 N, Tropic ofCancer, the Sun appears directlyoverhead at noon. 90 – 23 (or 67 N), Article Circle,the Sun does not set that day.
Equinoctia Halfway between the solstices,on about March 21 andSeptember 21. The Sun is on the celestialequator and whole planetreceives 12 hours day/night At the poles of the Earth, theSun only rises or sets once peryear on the equinox.
Length of the Day A day is a measure of how long the Earth takes to rotate relative to Relative to the Sun: Solar Day (24 hours) Relative to distance stars: Sidereal Day (23 hours 56 minutes)
Now:ScorpiusNightDaySummer: JulyNightDayOrionWinter: January13,000 years from now:ScorpiusNightDayWinter: July or January?DayNightSummer: January or July?We choose to keep July a summer month, but then in 13,000 years, summer occurs onother side of orbit!Orion
The YearSummerNightWinterDaySun high innorthern skySun low in Daynorthern skyNight The Earth revolves around the Sun in 365.256 days (“sidereal year”). But the year we use is365.242 days (“tropical year”). Why? Why do we have a leap year?
The Motion of the Moon Half of the Moon's surface is lit by the Sun. The Moon has a cycle of "phases", which lasts about 29 days. During this cycle, we see different fractions of the sunlit side.Which way is theSun here?
Cycle of phases slightly longer than time it takes Moon to do a complete orbitaround Earth.Cycle of phases orOrbit time or"synodic month”"sidereal month”29.5 days27.3 days
Tides A feature of oceans (but solid materials have smalltides too). Two high and two low tides per day. Tides are due to the gravitational pull beingstronger on side of Earth closest to it (Sun causessmaller tides) Earth-Moon gravity keeps them orbiting eachother. But side of Earth closest to Moon hasslightly stronger pull to Moon bulges towards it. Other side has weaker pull bulges awaycompared to rest of Earth. The Earth spins once a day while the bulge alwayspoints towards and away from the Moon highand low tides.
EclipsesLunar eclipse: When the Earth passes directly between the Sun and theMoon.EarthMoonSunSolar eclipse: When the Moon passes directly between the Sun and the Earth.MoonSunEarth
Moon's orbit tilted compared to Earth-Sun orbital plane:SunMoonEarth5.2oSide viewMoon's orbit slightly elliptical:MoonDistance varies by 14%EarthTop view, exaggerated ellipse
Earth is actually its closest to the Sun in January. Earth varies its distance to the Sun by about 3% over a year. Conclusion: Earth changing its distance to the Sun create seasons. Axial tilt, 23.5o, does create seasons. The Suns position in the sky changes throughout the year because of the tilt
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Moon Orbits the Earth The moon revolves (orbits) the Earth every 27.3 days and rotates on its axis every 27.3 days. This causes the same side of the moon to always face the earth. Phases of the moon: New moon – Waxing Crescent – 1st Quarter – waxing gibbous – Full moon – Waning gibbous – 3rd Quarter – Waning Crescent - New Moon
Like the sun, the moon rises and sets. It rises in the eastern sky. Then it sets in the west. During the time that the moon is . visible, it appears to move across the sky. Unlike the sun, the moon actually moves! It moves in a path around Earth. Earth also spins. So the moon’s place in the sky changes. Can you tell if the . moon is rising or .
Section 3: Exploring Earth's Moon Section 1: Earth Section 2: The Moon - Earth's Satellite. Chapter 23: The Sun-Earth-Moon System Section 1: Earth Grade 6 Earth Science . view at daybreak. Earth continues to spin, making it seem as if the Sun moves across the sky until it sets at night.
