The sun also exhibits an optical illusion at
sunrise similar to the moon illusion.
The apparent westward
revolution of Sun around the earth after rising out of the horizon is due to
Earth's eastward rotation, a counter-clockwise revolution when viewed from
above the North Pole. This illusion is so convincing that most cultures had
mythologies and religions built around the geocentric model. This same effect
can be seen with near-polar satellites as well.
Sunrise and sunset are
calculated from the leading and trailing edges of the Sun, and not the center;
this slightly increases the duration of "day" relative to
"night". The sunrise equation, however, is based on the center of the
sun.
The timing of sunrise
varies throughout the year and is also affected by the viewer's longitude and
latitude, altitude, and time zone. Small daily changes and noticeable
semi-annual changes in the timing of sunrises are driven by the axial tilt of
Earth, daily rotation of the earth, the planet's movement in its annual
elliptical orbit around the Sun, and the earth and moon's paired revolutions
around each other. In the springtime, the days get longer and sunrises occur
earlier every day until the day of the earliest sunrise, which occurs before
the summer solstice. In the Northern Hemisphere, the earliest sunrise does not
fall on the summer solstice around June 21, but occurs earlier in June. The
precise date of the earliest sunrise depends on the viewer's latitude
(connected with the slower Earth's movement around the aphelion around July 4).
Likewise, the latest sunrise does not occur on the winter solstice, but rather
about two weeks later, again depending on the viewer's latitude. In the
Northern Hemisphere, the latest sunrise occurs in early January (influence from
the Earth's faster movement near the perihelion, which occurs around January
3). Likewise, the same phenomena exist in the Southern Hemisphere except with
the respective dates reversed, with the latest sunrises occurring some time
after June 21 in winter and earliest sunrises occurring some time before
December 21 in summer, again depending on one's southern latitude. For one or
two weeks surrounding both solstices, both sunrise and sunset get slightly
later or earlier each day. Even on the equator, sunrise and sunset shift
several minutes back and forth through the year, along with solar noon. These
effects are plotted by an analemma.
Due to Earth's axial
tilt, whenever and wherever sunrise occurs, it is always in the northeast
quadrant from the March equinox to the September equinox and in the southeast
quadrant from the September equinox to the March equinox. Sunrises occur due
east on the March and September equinoxes for all viewers on Earth.
Colors
Incident solar white
light traveling through the Earth's atmosphere is attenuated by scattering and
absorption by air molecules and airborne particles via a combination of
Rayleigh scattering and Mie scattering. At sunset and sunrise, sunlight's path
through the atmosphere is much longer than during the daytime, which creates
different colors. At sunrise and sunset, there is more attenuation and light
scattering by air molecules that remove violets, blues, and greens, relatively
enhancing reds and oranges. Because the shorter wavelength light of violets,
blues, and greens scatter more strongly by Rayleigh Scattering, violets, blues,
and greens are removed almost completely from the incident beam, leaving mostly
only longer-wavelength orange and red hues at sunrise and sunset, which are
further scattered by Mie scattering across the horizon to produce intense reds
and oranges when there are soot, dust, or solid or liquid aerosols in the
atmosphere. The removal of the shorter wavelengths of light is due to Rayleigh
scattering by air molecules and small particles of sizes an order of magnitude
smaller that the wavelength of visible light (typically particles and molecules
smaller than 50 nm). The sun is actually white when observed without any air
between the viewer and the sun, so, sunlight in outer space contains a mixture
of violets, blues, greens, yellows, oranges, and reds. Due to Rayleigh scattering,
the sun appears reddish or yellowish when we look at it from Earth, since the
longer wavelengths of reds and yellow light are scattered the least, passing
through the air to the viewer, while shorter wavelengths like violet, blue, and
green light are in effect removed from direct sunlight by air molecules'
Rayleigh scattering.
Rayleigh scattering is
the elastic scattering of electromagnetic radiation due to the polarizability
of the electron cloud in molecules and particles much smaller than the wavelength
of visible light. Rayleigh scattering intensity is fairly omnidirectional and
has a strong reciprocal 4th-power wavelength dependency, and, thus, the shorter
wavelengths of violet and blue light are affected much more than the longer
wavelengths of yellow to red light. During the day, this scattering results in
the increasingly intense blue color of the sky away from the direct line of
sight to the Sun, whereas, during sunrise and sunset, the much longer path
length through the atmosphere results in the complete removal of violet, blue,
and green light from the incident rays, leaving weak intensities of orange to
red light.
After Rayleigh
scattering has removed the violets, blues, and greens, people's viewing of red
and orange colors of sunsets and sunrises is then enhanced by the presence of
particulate matter, dust, soot, water droplets (like clouds), or other aerosols
in the atmosphere, (the notable one being sulfuric acid droplets from volcanic
eruptions). Particles much smaller than the wavelength of the incident light
efficiently enhance the blue colors for off-axis short path lengths through air
(resulting in blue skies, since Rayleigh scattering intensity increases as the
sixth power of the particle diameter). Larger particles as aerosols, however,
with sizes comparable to and longer than the wavelength of light, scatter by
mechanisms treated, for spherical shapes, by the Mie theory. Mie scattering is
largely wavelength-insensitive. Its spacial distribution is highly preferential
in the forward direction of the incident light being scattered, thus having its
largest effect when an observer views the light in the direction of the rising
or setting Sun, rather than looking in other directions. During the daytime,
Mie Scattering in general causes a diffuse white halo around the Sun,
decreasing the perception of blue color in the direction toward the Sun, and it
causes daytime clouds to appear white due to white sunlight. At sunset and
sunrise, Mie scattering off of particles and aerosols across the horizon, then
transmits the red and orange wavelengths that remain after Rayleigh scattering
has depleted the blue light. This explains why sunsets without soot, dust, or
aerosols are dull and fairly faint-red, while sunsets and sunrises are
brilliantly intense when there are lots of soot, dust, or other aerosols in the
air.
Sunset colors are
typically more brilliant than sunrise colors, because, in general, the evening
air contains more particles and aerosols and clouds than morning air. Cloud
droplets are much larger than the wavelength of light, so they scatter all
colors equally by Mie scattering, which makes them appear white when
illuminated by white sunlight during the daytime. The clouds glow orange and
red due to Mie scattering during sunsets and sunrises, because they are
illuminated with the orange and red light that remains after multiple prior
Rayleigh scattering events of the light from the setting/rising sun.
Ash from volcanic
eruptions, trapped within the troposphere, tends to mute sunset and sunrise
colors, whereas volcanic ejecta lofted into the stratosphere (as thin clouds of
tiny sulfuric acid droplets) can yield beautiful post-sunset colors called
afterglows and pre-sunrise glows. A number of eruptions, including those of
Mount Pinatubo in 1991 and Krakatoa in 1883, have produced sufficiently high
stratospheric sulfuric acid clouds to yield remarkable sunset afterglows (and
pre-sunrise glows) around the world. The high-altitude clouds serve to reflect
strongly-reddened sunlight still striking the stratosphere after sunset, down
to the surface.
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