6.1: The Earth-Moon System

Tides

Ocean tides are one very observable result of the Moon’s gravity acting on Earth. Figure \(\PageIndex{4}\) is a simplified drawing of the Moon’s position relative to the tides. Because water easily flows on Earth’s surface, a high tide is created on the side of Earth nearest to the Moon, where the Moon’s gravitational pull is strongest. Why is there also a high tide on the opposite side of Earth? The answer is that Earth is pulled toward the Moon more than the water on the far side, because Earth is closer to the Moon. So the water on the side of Earth closest to the Moon is pulled away from Earth, and Earth is pulled away from water on the far side. As Earth rotates, the tidal bulge (an effect of the tidal forces between an orbiting natural satellite and the primary planet that it orbits) keeps its orientation with the Moon. Thus there are two tides per day (the actual tidal period is about 12 hours and 25.2 minutes), because the Moon moves in its orbit each day as well).

Figure \(\PageIndex{4}\): The Moon causes ocean tides by attracting the water on the near side more than Earth, and by attracting Earth more than the water on the far side. The distances and sizes are not to scale. For this simplified representation of the Earth-Moon system, there are two high and two low tides per day at any location, because Earth rotates under the tidal bulge.

The Sun also affects tides, although it has about half the effect of the Moon. However, the largest tides, called spring tides, occur when Earth, the Moon, and the Sun are aligned. The smallest tides, called neap tides, occur when the Sun is at a \(90^{\circ}\) angle to the Earth-Moon alignment.

Figure \(\PageIndex{5}\): (a, b) Spring tides: The highest tides occur when Earth, the Moon, and the Sun are aligned. (c) Neap tide: The lowest tides occur when the Sun lies at \(90^{\circ}\) to the Earth-Moon alignment. Note that this figure is not drawn to scale.

Tides are not unique to Earth but occur in many astronomical systems. The most extreme tides occur where the gravitational force is the strongest and varies most rapidly, such as near black holes (see Figure \(\PageIndex{6}\)). A few likely candidates for black holes have been observed in our galaxy. These have masses greater than the Sun but have diameters only a few kilometers across. The tidal forces near them are so great that they can actually tear matter from a companion star.

Figure \(\PageIndex{6}\): A black hole is an object with such strong gravity that not even light can escape it. This black hole was created by the supernova of one star in a two-star system. The tidal forces created by the black hole are so great that it tears matter from the companion star. This matter is compressed and heated as it is sucked into the black hole, creating light and X-rays observable from Earth.

 

Earth-Moon System 

Synchronous rotation – time to complete orbit = time to complete rotation 

Tidally locked – 

 gravity of Earth pulls on Moon forming bulge,  

Tidal friction - Earth pulls on bulge as it rotates away 

has slowed down rotation of Moon over time until bulge permanently pointed at Earth 

Works both ways, Earth has slowed down as well,  

may eventually become locked to the Moon,  

Exercise: consequences of becoming tidally locked to Moon 

so only one side of Earth can see the Moon,  

Permanent tides – constant sea level 

Sun may become a red giant first 

Center of Mass 

Eclipses 2 –  

view from space plane of Moon’s orbit is tilted relative to orbit of earth around the Sun 

Line of nodes 

Saros cycle – needs long records of eclipses 

Tides 

Gravity review 

Shape of tides 

Spring, neap 

Rotation of earth 

Synchronous rotation 2 – tidal friction, also called tidally locked 

Tidal effect of Moon on Earth 

2.  

Learning Objectives

By the end of this section, you will be able to:

• Describe the components of Earth’s interior and explain how scientists determined its structure
• Specify the origin, size, and extent of Earth’s magnetic field

Earth is a medium-size planet with a diameter of approximately 12,760 kilometers (Figure 8.2). As one of the inner or terrestrial planets, it is composed primarily of heavy elements such as iron, silicon, and oxygen—very different from the composition of the Sun and stars, which are dominated by the light elements hydrogen and helium. Earth’s orbit is nearly circular, and Earth is warm enough to support liquid water on its surface. It is the only planet in our solar system that is neither too hot nor too cold, but “just right” for the development of life as we know it. Some of the basic properties of Earth are summarized in Table 8.1.

Figure 8.2 : Blue Marble. This image of Earth from space, taken by the Apollo 17 astronauts, is known as the “Blue Marble.” This is one of the rare images of a full Earth taken during the Apollo program; most images show only part of Earth’s disk in sunlight. (credit: modification of work by NASA)

Table 8.1: Some Properties of Earth

Property	Measurement
Semimajor axis	1.00 AU
Period	1.00 year
Mass	$5.98\times {10}^{24}\text{kg}$
Diameter	12,756 km
Radius	6378 km
Escape velocity	11.2 km/s
Rotational period	23 h 56 m 4 s
Surface area	$5.1\times {10}^8{\mathrm{km}}^2$
Density	5.514 g/cm^3
Atmospheric pressure	1.00 bar

Attributions

https://phys.libretexts.org/Bookshel...al_Perspective

https://phys.libretexts.org/Bookshel...of_Gravitation