# Earth 🜨

• ~13,000 kilometers in diameter
• The Moon is the nearest astronomical neighbour
• The Sun is about 150 million kilometers away
• Axis of rotation is tilted 23.5˚
• This is why the Sun is visible for less time in the winter and more time in the summer (in the northern hemisphere)
• Composed primarily of heavy elements

## Layers

• Separated into layers of material based on different densities

### Mantle

• Largest part of the solid Earth
• Oozing flow (like lava)
• Stretches from base of the crust --> 2900 km deep
• ~84% of Earth's volume

### Core

• 5000 K
• 1.4 million atmospheres
• High pressure
• Diameter of 7000 km
• Larger than the entire planet Mercury
• Outer core is liquid
• 1200 km
• Inner core is probably solid

## Magnetic Field and Magnetosphere

• Magnetic field generated by moving material in the liquid metallic core
• Extends into surrounding space
• Magnetosphere
• The zone within which Earth's magnetic field dominates over the weak interplanetary magnetic field that extends outward from the Sun
• Extends ~60,000 km in the direction of the Sun
• Can reach as far as the orbit of the Moon on the side opposite the Sun

## Atmosphere

### Greenhouse Effect

• Trapping of infrared rays
• Visible light comes through, converted to heat, and then cannot escape

## Coordinate System

• Axis of rotation defines poles
• Equator is halfway between both poles
• East/west defined by Earth's rotation
• East --> direction toward which Earth rotates
• West --> opposite of East

## Calculating Earth's Size in Ancient times

• This means at Alexandria, Earth's surfaced has curved by 7˚ (1/50 of a full circle)
• The distance between the two cities must then be 1/50 the Earth's circumference, so we can measure that

## Orbit

• Distance from the Sun only varies by ~3%

## Seasons

• Caused by the tilt of Earth's axis (23.5˚)
• Depending where the Earth is in its orbit, one of the hemispheres will be more directly illuminated
• Sunlight becomes more/less effective at heating the Earth
• Tilting towards the Sun provides more direct sunlight
• Longer days gives the Sun more time to heat up the Earth
• The Sun will actually spend more/less time above the horizon

## Time

• Solar days are defined in terms of the Earth's rotation with respect to the Sun
• How we tell time
• ~4 minutes longer than a sidereal day ($\frac{1}{365}$ of a day)
• Stars rise 4 minutes earlier every day
• Sidereal days are defined in terms of the Earth's rotation with respect to the stars
• Stars rise at the same sidereal time every day
• Better for observing the stars
• Apparent solar time
• Time determined by the actual position of the Sun
• Time shown on sundials
• Not very convenient for us
• First half of the day, the Sun has not reached the meridian
• Before the Sun reaches the local meridian
• ante meridiem or a.m.
• Second half of the day, after noon
• post meridiem or p.m.
• After the Sun reaches the local meridian
• Mean solar time
• Average value of the solar day over the year
• Exactly 24 hours in a day
• Progresses at a uniform rate
• Standard time
• 24 standardized global time zones
• Each zone keep the same standard time with the local mean solar time running through the middle of each zone
• Travelers change their watches only when time change is >= one hour

## Tidal Force

• Not a real force like gravity
• The Moon pulls on the Earth, making tidal bulges in the oceans
• Water on the side facing the Moon flows toward it
• Water on the side away from the Moon has weak influence so tides are high
• Bulges out on the side facing the Moon as well as the side facing away from it
• This causes two high/low tides per day
• Once when away from the Moon, and once when facing it