1. Earth and its Moon 2 © Zanichelli editore 2015

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Earth and its Moon

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© Zanichelli editore 2015

Physical dataand the shape of Earth

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The measures of Earth

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Earth is a celestial body with a relatively small mass and volume, but high density.

The average surface temperature of 15 °C keeps most water in the hydrosphere in a liquid state.

Earth

Mass 5.974 * 1024 kg

Volume 1.083 * 1021 m3

Average density 5515 km/m3

Average radius 6371 km

Equatorial radius 6378 km

Polar radius 6357 km

Average surface temperatue 15 °C


The shape of Earth

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Earth rotates and has an almost spherical shape flattened at its poles, called geoid.

Geographical elements of reference are the rotational axis, the N and S poles and the equator.

Equator

North pole

South pole


Geographical coordinates

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The position of a point P on Earth’s surface is determined by latitude and longitude, called geographical coordinates.


Cardinal points and the horizontal plane

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The four cardinal points of the compass (North, South, East, West) can be identified on the horizontal plane, bounded by the apparent horizon.


Earth’s movements

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The rotation motion

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Earth rotates from West to East in a counter-clockwise direction.The rotation period is called a sidereal day, lasting 23h 56’ 4’’.

The linear speed of rotation is greatest at the equator and zero at the poles; centrifugal force depends on this value.


Day and night

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Day and night alternate at each point on Earth’s surface.Only at the equator do they have the same duration throughout the year.

The light zone is separated from the dark zone by the terminator, where, due to the atmosphere, the gradual passage from light to dark (and vice versa) occurs.


The revolution motion

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Earth completes an elliptical orbit around the Sun in a sidereal year, which corresponds to 365d 6h 9’ 9.5’’.Earth’s axis is inclined at 66° 33’ with respect to the plane of the ecliptic and it is always stable as Earth moves.


Spring and autumn equinoxes

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In spring and autumn equinoxes, day and night have the same duration everywhere and the Sun is at the zenith of the equator.


Winter solstice

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In winter solstice: Sun at the zenith at the Tropic of Capricorn.


Summer solstice

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In summer solstice: Sun at the zenith at the Tropic of Cancer.


The astronomical seasons

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The astronomical seasons are the amounts of time that pass between a solstice and an equinox, or vice versa.

They are characterized by variations in the height of the Sun above the horizon and by the day’s duration.

They have different effects on the polar icecaps, in the temperate zones and in the inter-tropical zones.


Astronomy and the measurementof time

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The solar day

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The solar day lasts 4 minutes longer than the sidereal day: 24h 00’ 4’’.

For this reason, the Sun has an apparent movement through the celestial sphere which is slower than the stars; in the course of a year it passes through the constellations of the zodiac.


Local time and the time zones /1

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Earth’s surface is divided into 24 time zones.

Each time zone consists of 15 meridians wherein local time is determined by the astronomical time of the middle of the meridian.


Local time and the time zones /2

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Going East from the Greenwich meridian, we add the same number of hours as the number of the time zone. Going West from Greenwich, we take away the same number.


The solar year, calendar year and sidereal year

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The solar year is the period between two equinoxes or solstices of the same season; it lasts 365 d 5h 48’ 46’’, which is rounded down to 365 days in the calendar year.

In order to keep them synchronized, in the western calendar one day is added every 4 years.

The sidereal year is the time Earth actually takes to complete one revolution around the Sun (365d 6h 9’ 9.5’’).


The Moon and its movements

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Close but different: Earth and Moon

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The Moon is the only terrestrial satellite. It differs from Earth: it lacks atmosphere, liquid water and vapor; it does not show geological activities or modeling of the surface. There is no life on the Moon.

Moon

Mass 7.3 * 1022 kg

Radius 1738 km

Density 3.34 g/cm3

Rotation period 27 days

Revolution 27 days

Translation period 365 days


The Moon’s movements

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The Moon has three main movements:

•rotating on its axis;•revolving around Earth;•orbiting the Sun together with Earth.

Rotation and revolution occur in the same direction and have the same duration; a sidereal month lasts 27 Earth days 7h 43’ (so does the lunar day).


The Moon’s phases /1

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In its revolution around Earth, the Moon displays four phases of illumination:• new Moon;• first quarter;• full Moon;• last quarter.

The lunar month is the period of time required for the Moon to complete its phases and lasts 29 d 12h 44’ 3’’.


The Moon’s phases /2

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Eclipse of the Moon

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Eclipses of the Moon take place when the Moon is full and is located on or near an orbital node, and Earth casts a shadow over it.


Eclipse of the Sun

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Eclipses of the Sun occur when the Moon is full and on or near one of the orbital nodes, blacking out the solar surface.


Documents

1. Earth and its Moon 2 © Zanichelli editore 2015