30.6 - Earth's seasons
Almost all the energy we use on Earth originates in the Sun and arrives in the form of
electromagnetic radiation. Roughly the same amount of solar power reaches the planet
throughout the year, yet many places on the globe experience significant seasonal
variations in the rate at which they receive this energy.
The cause of seasonal changes in the Earth’s climate is the tilt of its axis, the line about
which it rotates. The illustration in Concept 1 shows the position of the Earth in its orbit
at different times of the year, as well as the direction in which the axis points. The axis
is tilted at a 23.5° angle away from a line perpendicular to the Earth’s orbital plane. It
always points towards the same direction in space (which is why Polaris remains the
North Star throughout the year).
March 21 and September 22 are known as the equinoxes. The name refers to the equal
lengths of night and day (12 hours each) for all locations on Earth on these dates.
December 21 and June 21 are the solstices. As the season progresses from autumn to
winter, the Sun rises to a lower high point in the sky each day, and the days get shorter.
On the winter solstice (meaning “sun stop”), the Sun stops getting lower and begins to
rise to a higher apex each day, as it does through the rest of the winter and spring. The
opposite happens after the summer solstice í the Sun once again peaks at a lower
point each day. (The dates of the equinoxes and solstices vary from year to year, but
are always around the 21st of the month.)
While June 21 is the summer solstice in the Northern Hemisphere, it is the winter
solstice for the Southern Hemisphere. Concept 2 illustrates why this is true. It shows a
Northern Hemisphere city, Beijing, and a Southern Hemisphere city, Perth, at noon on
June 21.
Imagine a solar collection plate of area A lying flat on the ground, tangent to the Earth’s
surface in either of these cities. Light rays from the Sun arrive approximately parallel to
the plane of the Earth’s orbit. On June 21, the sunlight intersects the collecting plate in
Beijing at a steeper angle than in Perth. Because Beijing is receiving sunlight more
vertically, the energy from that light is more concentrated í Beijing is receiving more
power over the area of its collecting plate.
In Perth, a smaller amount of sunlight is being spread over the same collecting area
because of the oblique, slanting angle at which it hits the plate. The plate absorbs less
power. It is summer in Beijing, and winter in Perth.
Six months later, on December 21, the situation will be reversed: Perth will receive more direct sunlight than Beijing.
Generally speaking, locations farther than Beijing or Perth from the equator experience a greater variation in the power they receive throughout
the year, and places closer to the equator experience less change. At the poles í dark six months of the year í this difference is extreme.
Some people mistakenly believe that the seasons are due to the eccentricity of the Earth’s orbit í the fact that the Earth’s distance from the
Sun changes throughout the year. Your first clue that this belief is false is the observation that summer in the Southern Hemisphere occurs at
the same time as winter in the Northern Hemisphere (you merely have to make a long-distance phone call to confirm this). In fact, during winter
in the Northern Hemisphere, the Earth is actually closer to the Sun than in summer. The reason the eccentricity has only a slight effect is that
the Earth’s orbit is only slightly elliptical. The annual variation in insolation due to the eccentricity of the Earth’s orbit is about 7%, in contrast to
an approximately 110% increase from winter to summer (at the latitude of Beijing) due to axial tilt.
Cause of varying intensity
Earth’s axis of rotation is tilted
The seasons
Greater intensity in summer, less in
winter
Northern, Southern Hemispheres have
opposite seasons30.7 - How electromagnetic waves travel through matter
Light and other forms of electromagnetic radiation can travel through a vacuum, and it is
often simplest to study them in that setting. However, radiation can also pass through
matter: If you look through a glass window, you are viewing light that has passed
through the Earth’s atmosphere and the glass. Other forms of radiation such as radio
waves pass through matter, as well.
This section focuses on how such transmission occurs. It relies on a classical model of
electrons and atoms that predates quantum theory. In this model, electrons orbit an
atom. They have a resonant frequency that depends on the kind of atom. On a larger
scale, atoms themselves and the molecules composed of them also have resonant
thermal frequencies at which they can vibrate or rotate.
We will use the example of light striking the glass in a window to discuss how
substances transmit (or do not transmit) electromagnetic radiation. When an
electromagnetic wave encounters a window, it collides with the molecules that make up
the glass. If the frequency of the wave is near the resonant thermal frequency of the
glass molecules, which is true for infrared radiation, the amplitude of the molecules’
vibrations increases. They absorb the energy transported by the wave, and dissipate it
throughout the glass by colliding with other molecules and heating up the window.
Because it absorbs so much infrared energy, the glass is opaque to radiation of this frequency, preventing its transmission.
Glass opaque to infrared
Radiation is in resonance with
molecules
Waves do not travel through