Earth Science: Revolution, Axis, and Axial Tilt

• Earth Science 2 discusses seasonal and daily temperatures in meteorology lectures.

Earth's Revolution

• Earth orbits the sun in a path called an orbit, which is slightly oval-shaped.
• It takes 365.25 days for Earth to complete one revolution around the sun, traveling 940 million kilometers.
• Earth orbits the sun at approximately 30 kilometers per second while simultaneously rotating on its axis.

Axis

• An axis is an invisible line around which an object rotates, intersecting the North and South Poles on the object's surface.
• The rotation period, or the time it takes for a planet to complete one spin around its axis, is about 24 hours, or one day, on Earth.

Axial Tilt

• Earth's axis has an axial tilt, or obliquity, rather than being perpendicular.
• Axial tilt is the angle between a planet's rotational axis and its orbital axis, and is 23.5 degrees for Earth.
• This tilt causes the sun to shine on different latitudes at varying angles throughout the year, creating seasons.

Axial Precession

• Earth's axis wobbles slowly like a spinning top, completing a circular wobble, called axial precession, in approximately 26,000 years.
• The axis helps determine the North Star, and its precession changes it; Polaris is the current North Star since it's directly above the North Pole.
• In 13,000 years, the Earth's axis will wobble away from Polaris, pointing toward Vega as the new North Star.

Earth's Rotation

• Rotation happens when an object turns around an invisible line down its center, which is the vertical axis from the North Pole to the South Pole for Earth.
• Earth takes about 24 hours to make a complete rotation.
• The Earth rotates unevenly.
• Earth rotates around the Equator at about 1,670 kilometers per hour, and at 45° north, it’s 1,180 kilometers per hour.
• Rotation creates the periods of light and darkness known as day and night.
• The half of Earth facing the sun is in daylight, and the opposite half is in darkness.
• If Earth did not rotate, half would be too hot for life while the other half would be frozen.
• Earth rotating from west to east makes the sun rise in the east and set in the west.

Earth-Sun Relationship

• The amount of light that it receives from the sun leads to the seasons and biodiversity on Earth.
• A region's sun exposure is based on Earth's axial tilt rather than its distance from the sun.
• The Northern Hemisphere has summer in June, July, and August because the sunlight is more direct because of the tilt.
• From December to February, the Southern Hemisphere tilts to face the sun.

Distance to the Sun

• Earth's average distance from the sun is roughly 150 million kilometers, or one astronomical unit (1 AU).
• According to the International Astronomical Union (IAU), one astronomical unit (AU) equals 149,597,871 kilometers.
• Because Earth's orbit around the sun is elliptical, the distance between them is not constant.
• An astronomical unit is a practical approximation of this distance.

Seasons

• A season is a period distinguished by climate conditions like spring, summer, fall, and winter.
• Four seasons follow one another, and weather patterns repeat yearly.

Causes of the Seasons

• Earth's tilted axis causes the seasons.
• The sun's most direct rays are received by different parts of Earth throughout the year.
• It determines the seasons.
• Summer in the Northern Hemisphere occurs when the North Pole faces the Sun.
• Winter season occurs in the Northern Hemisphere when the South Pole faces the sun.
• The Earth's tilt is the reason for differences in the seasons.
• The Northern Hemisphere receives direct sunlight and warmth causing spring and summer when tilted to the sun.
• The cooling from less sunlight means autumn and winter when tilted away from the sun.
• Because of Earth’s 23.5° tilt, seasons are reversed in the Northern and the Southern Hemisphere, about six months apart.
• The North Pole points towards the Sun (around June), and the South Pole faces towards it (around December).
• The seasons are opposite on sides, with summer in June in the Northern Hemisphere because sun rays strike it more.
• Winter is in December in the Northern Hemisphere, when the South Pole tilts toward the Sun.
• Earth has seasons due to its axis being tilted, yet it always points in the same direction while rotating the sun.
• The Earth got its tilt from a collision with an object named Theia when Earth was younger.
• The amount of debris flung out became the Moon. Its tilted axis always points the same way.

Sun Intensity vs. Angle

• Sun intensity is the amount of solar energy reaching Earth's surface, determined by the angle of the sun's rays; it varies by location, time of year and time of day.
• Locations along one latitude on Earth could potentially receive sunlight at a 90-degree angle on a given day, so they would get high intensity sunlight.
• In general, the sun's rays are most intense at the Equator and least intense at the poles.

Angle of Incidence

• The angle of incidence is technically the angle formed by sunlight hitting the Earth.
• Rays striking the surface from directly overhead (measured at a 90-degree angle from the horizon) are most intense.
• The smaller the angle, the less intense is the radiation, which spreads over a larger surface area.
• E.g., at a 45-degree angle incidence, solar radiation has 40 percent more area, and is 30 percent less intense than at 90 degrees.

Latitudinal Variation

• Only places at the same latitude get sun at a 90-degree angle on any given day.
• Sun intensity is greatest at the equator and least at the poles.
• Areas north of the Arctic Circle only get about 40% as much sun as equatorial areas on yearly average.

Relationship to Seasons

• The intensity and length of solar energy determine seasons because they’re how the Earth is positioned.
• Earth tilts at a 23.5degree angle, so the Northern Hemisphere faces the sun more than the Southern Hemisphere and vice versa.
• For instance, on the summer solstice, the sun touches 23.5 degrees north, the Tropic of Cancer.
• A larger percentage of solar radiation is received to whichever hemisphere tilts toward the sun.
• As the world goes through the summer, with each ray that shines there the sky will become brighter and more intense.
• Hence, rates of sunburn usually increase during summer.

Earth's Distance vs. Seasons

• Many believe Earth is closer to the sun during summer, however Earth's varying distance from the sun does not cause seasons.
• Although Earth's orbit isn't a perfect circle, the Northern Hemisphere is in winter when the Earth is closest to the sun and summer when it's farthest.

Distance from the Sun

• Earth's perihelion is the point when it's closest to the sun: 91,400,000 miles (147 million kilometers) away.
• Earth's aphelion is the point when it's farthest from the sun: 94,500,000 miles (152 million kilometers) away.
• There’s about a 7 percent difference between solar activity at the aphelion and perihelion.
• The aphelion for the Earth actually occurs in July, and the parihelion in January.

Solstice and Equinox

• Equinoxes occur when parts of the Earth experience equal day and night.
• Solstices are days with the maximum (summer solstice) and minimum (winter solstice) amount of daylight.
• The intensity of the sun's rays over a given region is one of the key factors determining the seasons, which changes as Earth revolves.
• In the Northern Hemisphere the shortest day is on either December 21 or 22.
• The longest day is from June 20-21.
• Spring and Autumn are days that equal amount of daylight and darkness.

Seasonal Timing

• In the Northern Hemisphere:
• Winter starts on December 21 or 22;
• Summer starts on June 20 or 21, and are astronomical events.
• Meteorological seasons are defined as: Winter (December 1 to February 28/29), Spring (March 1 to May 31), Summer (June 1 to August 31), and Fall (September 1 to November 30).
• The vernal or spring equinox is on March 20 or 21.
• The autumnal equinox is on September 22 or 23.
• If Earth's axis were perpendicular to its orbit, it would not experience seasonal variations because tilt is responsible.
• Without the axial tilt, Earth would experience consisten sunlight exposure and consistent amount of daylight, year-round.
• Temperature conditions near the equator would continue the same with no change.

Milankovitch Cycles

• Milankovitch cycles help determine short and long-term climate.
• The idea that the Earth's relative location to the sun has effects on Earth and causes ice age effects was hypothesized by Milutin Milankovitch.
• Earth orbital movements affecting solar radiation reaching Earth are Milankovitch cycles, which impact 25 percent of Earth.
• 30 to 60 degrees latitude north and south of the equator are affected by Milankovitch cycles.
• Three parts to them are orbit shape (eccentricity); axis tilt (obliquity); and axis direction (precession).

Eccentricity

• Earth's pilgrimage around the Sun isn't exactly circular, but over time, the gravity from Jupiter and Saturn cause it to vary from nearly circular to slightly elliptic.
• Eccentricity measures the deviation of Earth's orbit from a perfect circle to each other.
• The eccentricity causes summers in the North Hemisphere to be 4.5 days longer than winters, and springs to be three days longer than autumns.
• Eccentricity decreases, and each each season evens out.
• The difference in distance between the Sun at its closest approach (perihelion) around Jan 3 and its farthest departure (aphelion) around July 4 is 5.1 million km equal to 3.4 percent.
• In January there willl be 6.8 percent incoming solar radiation.
• 23 percent more incoming solar radiation when Earth's orbit is at most elliptic vs its farthest departure, and Earth's eccentricity is slowly approaching least elliptic.

Obliquity

• Earth's axis is tilted while revolving around the sun, known as obliquity, which causes Earth to have its seasons.
• Earth has been between 22.1 and 24.5 degrees for the last million years and it occurs with respect to Earth's orbital plane.
• As each hemisphere recieves more solar radiation, the more extreme the tilt is and they favour deglaciation.
• Effects are not spread evenly globablly higher lattiudes get bigger solar changes.
• Earth’s axis is at 23.4 degrees, slowly decreasing in a cycle spanning about 41,000 years, and was last at its maximum 10,000 years ago.
• When obliquity increases, seasonal contrasts become more extreme, meaning colder winters and hotter summers.

Precession

• As Earth rotates, Earth wobbles like a toy, and tidal forces from the sun and the moon cause its equator to bulge out.
• The wobble is known as axial precession that spans about 25,771.5 years.
• Axial precession has the most extreme effects on seasonal contrasts in each hemisphere.
• The wobble affects season extremes, solar patterns and so has the most extreme variation in solar cycles between summer and winter.

Daily Temperature Variations

• The diurnal cycle is the shift in temperature that is related to rotation of Earth from day to night.
• Day and night also affect the temperature changes on surface with day and night. Heating and cooling.
• Solar, terrestrial imbalances lead to temperature changes.
• Surface warms during day, and cools at night.
• Temperature changes are affected across globe by latitude, season, climate and weather.
• Highest temperature occurs when incoming solar radiation equals outgoing radiation between 3 to 5 pm local time.
• There is a lag that occurs.
• The lowest temperatures occur when heat is lossed into space before sunrise
• Air near the surface heats or cools depending on various causes.

Additional Info

• The max temperature for a day is highest during recording of hours.
• Rising of Sun affects temperature and rays extend back.
• Land must absosrb the rays.
• High energy leads to high temperatures, high solar radiation and light.
• The thin layer on Earth can cool through conduction
• Warmer due to less dense air than cooler air.
• Minimum temperature of day refers to lowest recorded for 24 hour time.
• With direct solar radiation, Earth becomes cooling and it will lose solar radiation, depending on climate, weather patterns and the time of shift.
• Ground having absorbed the heat, then is released.
• Air contact cools.