Astronomy-Part 10 Notes The Earth-Moon-Sun Systems

Days Go By… Each planet spins on its axis. The spinning of a body, such as a planet, on its axis is called rotation. The time it takes a planet to complete one full rotation on its axis is called a day. Earth rotates in a counterclockwise motion around its axis when viewed from above the North pole. As a location on Earth’s equator rotates from west to east, the sun appears to rise in the east, cross the sky, and set in the west. Only one-half of Earth faces the sun at any given time.

Days Go By… People on the half of the Earth facing the sun experience daylight, and this period is called daytime. People on the half of the Earth facing away from the sun experience darkness, and this period is called nighttime. Earth completes one rotation on its axis in 24 hours, or one day.

Another Year Older… As Earth rotates on its axis, it also revolves around the sun. The motion of a body that travels around another body in space is called revolution. Earth completes a full revolution around the sun in 365.25 days, or about one year. We have divided the year into 12 months, each lasting 28 to 31 days.

Earth’s Tilt Earth’s axis is tilted at 23.5°. During each revolution, the North Pole may be tilted toward the sun or away from the sun. When the North Pole is tilted toward the sun, the Northern Hemisphere has longer periods of daylight than the Southern Hemisphere has. When the North Pole is tilted away from the sun, the opposite is true.

Earth’s Tilt The angle at which the sun’s rays strike each part of Earth’s surface changes as Earth moves in its orbit. When the North Pole is tilted toward the sun, the sun’s rays strike the Northern Hemisphere more directly, and this region is warmer. When the North Pole is tilted away from the sun, the sun’s rays strike the Northern Hemisphere less directly, and this region is cooler.

Earth’s Tilt The spherical shape of Earth also affects how the sun warms up an area. At a point near the equator, the sun’s rays hit Earth’s surface more directly, so temperatures are higher there. At a point near one of the poles, the sun’s rays hit Earth’s surface less directly, so temperatures are lower there.

Earth’s Tilt The number of daylight hours on any given day of the year varies with location. Areas around Earth’s equator receive about 12 hours of light a day. Areas on Earth’s surface that are tilted toward the sun have longer than 12-hour days, as they travel a longer path through the lit part of Earth. Areas on Earth’s surface that are tilted away from the sun have shorter than 12-hour days, as they travel a shorter path through the lit part of Earth.

Earth’s Tilt During summer in the Northern Hemisphere, areas north of the Arctic Circle receive 24 hours of daylight. At the same time, areas south of the Antarctic Circle receive 24 hours of darkness. In winter in the Northern Hemisphere, the polar areas experience the reverse conditions.

‘Tis the Season(s)… Most locations on Earth experience seasons. Each season is characterized by a pattern of temperature and other weather trends. We experience seasons due to changes in the intensity of sunlight and the number of daylight hours as Earth revolves around the sun. At an equinox, sunlight shines equally on the Northern and Southern hemispheres. Half of each hemisphere is lit, and half is in darkness. As Earth moves along its orbit, the sunlight reaches more of one hemisphere than the other.

‘Tis the Season(s)… At a solstice, the area of sunlight is at a maximum in one hemisphere, and at a minimum at the other hemisphere. During the September equinox, sunlight shines equally on both poles. During the March equinox, half of each hemisphere is lit, and the sunlight is centered on the equator.

‘Tis the Season(s)… At the December solstice, the North Pole leans away from the sun and is in complete darkness, and the South Pole is in complete sunlight. At the June solstice, the North Pole leans toward the sun and is in complete sunlight, and the south pole is in complete darkness.

The Earth-Moon System Earth spins on its axis and orbits around the sun. A body that orbits a larger body is called a satellite. Smaller bodies that travel around planets are natural satellites called moons. Gravity is the force that pulls all bodies that have mass toward other objects. Earth’s gravitational pull on the moon keeps the moon in orbit, forming the Earth-Moon system. The distance between the Earth and the moon is roughly 383,000 km (283,000 mi).

The Earth-Moon System The moon is only visible from Earth when it reflects sunlight. Only one side of the moon, often called the near side, faces Earth. The moon rotates once on its axis for every 28.5 days it takes to revolve around Earth.

It’s Just a Phase… As the moon revolves around Earth, the portion of the moon that reflects sunlight back to Earth changes. The lunar phases are changes in the moon’s appearance due to its position in orbit around Earth. Lunar phases cycle monthly, and begin with a new moon. The new moon is hard to see because Earth, the moon, and the sun are lined up, making the moon unlit.

It’s Just a Phase… As the moon moves in its orbit, it reflects more sunlight, and its crescent shape grows larger. The moon waxes, or grows, until half of the near side is in sunlight. This is the first quarter. The gibbous phase is when the near side is more than half-lit, but not fully lit. When the moon is fully lit, it is called a full moon. The lit portion of the moon shrinks, or wanes, during the third week of the cycle.

It’s Just a Phase… When the near side is only half-lit in sunlight, it is three-quarters through the cycle. The phase is called the third quarter. When the moon is seen as a waning crescent shape, the cycle is almost complete. When the moon is again unlit as a new moon, the cycle is complete.

Eclipses… An eclipse is an event during which one object in space casts a shadow on another object. A lunar eclipse occurs when the moon moves through Earth’s shadow. The umbra is the darkest part of a shadow. Around the umbra is spreading a cone of lighter shadow, called the penumbra.

Eclipses… Before a lunar eclipse, the moon is a full moon. The moon moves into the penumbra shadow and becomes less bright. When the moon moves into the umbra, the moon is in total darkness. A total lunar eclipse occurs when the moon moves completely inside the umbra. If the moon misses all or part of the umbra and part stays lit, it is called a partial lunar eclipse. You do not see lunar eclipses each month because the moon’s orbit is tilted by about 5° relative to Earth’s orbit.

Solar Eclipses When the moon is directly between the sun and Earth, the shadow of the moon falls on a part of Earth and causes a solar eclipse. (Must be a NEW MOON) When the sun’s light is completely blocked by the moon, it is a total solar eclipse. Outside the umbra, but within the penumbra, people see a partial solar eclipse. The moon’s umbra makes a shadow that is never more than a few hundred kilometers across. A total eclipse covers only part of Earth and can only be seen in particular areas. A total solar eclipse happens somewhere on Earth every one to two years.

Earth’s Tides Tides are daily changes in the level of ocean water. Tides are caused by the difference in the gravitational force of the sun and moon across Earth. The difference in gravitational force is called the tidal force. Because the moon is closer to Earth, the moon is mainly responsible for Earth’s tides. Water on the side of Earth closest to the moon bulges toward the moon. A bulge is created on both the near side and far side of Earth.

Earth’s Tides High tide is a water level that is higher than the average sea level. Low tide is a water level that is lower than the average sea level. Tidal bulges move around Earth following the motion of the moon.

Earth’s Tides The moon’s tidal force is greater than the sun’s tidal force, which results in different tidal ranges. Tidal range is the difference between the levels of ocean water at high tide and low tide.

Earth’s Tides Neap tides are tides that have the smallest daily tidal range. Neap tides happen when the sun, moon, and Earth form a 90° angle. During a neap tide, the gravitational effects of the sun and moon on Earth do not add together.

What Causes Tidal Cycles? The moon revolves around Earth much more slowly than Earth rotates. A place on Earth facing the moon takes 24 h and 50 min to face the moon again. So, the cycle of high tides and low tides at that place happens 50 min later each day. Because the tidal cycle occurs in 24 h and 50 min intervals, it takes about 6 h and 12.5 min for water in an area to go from high tide to low tide. It takes about 12 h and 25 min to go from one high tide to the next high tide.