Earth 🜨

  • ~13,000 kilometers in diameter
  • The Moon is the nearest astronomical neighbour Earth and Moon, drawn to scale
  • 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


  • Separated into layers of material based on different densities



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


  • 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

Earth's 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


Structure of the atmosphere

Atmospheric Layers

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

How the Earth's size was calculated

  • In Syene, the Sun was straight overhead (no shadows created) but in Alexandria had shadows with 7˚
  • 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


  • Distance from the Sun only varies by ~3%



  • 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 Sun's path in the sky depending on the season


Sidereal Day vs Solar Day

  • 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

Tidal bulges

  • 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

Space Astronomy