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Ten Things You Thought You Knew About Sun-Earth Science
A list of common and uncommon, famous and infamous misconceptions about solar-terrestrial physics...
- Earth is closest to the Sun is the summertime...or, it is warmer in summer
because Earth is closer to the Sun...
False. This is by far the most common misconception about the relationship between
Sun and Earth, one that is unfortunately perpetuated by lousy diagrams
in most school textbooks. When someone says we are closer to the Sun in
the summertime, do you ever remind them that while it may be summer in the
northern hemisphere, it is wintertime in the southern hemisphere...and six
months from now, the seasons and Earth's position will be reversed? In fact,
in summer in the northern hemisphere, Earth is actually at its farthest point
away from the Sun. So how can that be?
Despite what you learned in school about Earth's "elliptical" orbit around the Sun,
that elliptical orbit is pretty close to being circular (not the extended
oval you see in most books). The change of seasons is mainly the result of
the tilt of Earth's invisible spin axis, which is inclined 23.5 degrees in comparison
to the axis of the Sun. Sometimes Earth's axis is tilted toward the Sun and
sometimes away from it -- and somewhere in between for the rest of the year.
It is this tilt -- combined with the motion of the Earth around the Sun -- that
causes more or less light to fall on one hemisphere or the other during each of
the seasons. This means that the amount of sunlight falling directly on a parcel
of Earth changes throughout the year. It also means that days get longer or shorter,
causing the Sun to warm part of the Earth for longer and shorter periods of each day.
To learn more, visit
http://www.badastronomy.com/bad/misc/seasons.html
or
http://www-istp.gsfc.nasa.gov/stargaze/Sseason.htm
or
http://aa.usno.navy.mil/faq/docs/seasons_orbit.html
- The Sun does not rotate...
False. Like almost everything else in our clockwork universe, the Sun rotates around an
invisible axis. The rotation of the Sun was actually one of Galileo's greatest discoveries.
By observing sunspots moving across the face of the Sun, Galileo figured out that it wasn't
just the spots that were moving, but the whole star. It takes an average of 27 days for
the Sun to make a full rotation, though the rate is actually about 25 days at the equator
and as many as 32 days near the Sun's poles. This "differential" rotation -- where the equator
spins faster than the poles -- is perhaps the leading cause of the magnetic mayhem on the Sun
that leads to sunspots, flares, and coronal mass ejections.
- The Sun has a solid surface...
False. The song by Smash Mouth says, "You might as well be walking on the Sun."
Fat chance. The Sun is made entirely of gas, so there isn't a fixed, solid surface.
The part that looks like the surface -- the photosphere -- is simply the region of gas on
the edge of the Sun that emits light in wavelengths that we can see. (Did we mention
that you would be vaporized before you got close enough to walk on the Sun?)
- Space is empty, a complete vacuum...
False. Space is filled everywhere by plasma, the fourth state of matter
(solid, liquid, gas, and plasma). Plasma is a gas in which electrons
have been separated from their atoms (ions), making it electrically charged.
Plasma is extremely rare on Earth; you can only find it in candle flames,
lightning, and fluorescent lights. But in fact, 99% of the universe is made up of
plasma. Of course, space is so vast that all this plasma gets spread out
to a point where it seems like a vacuum by our earthly standards.
- The solar wind and coronal mass ejections (CMEs) can exert enough
pressure to push a satellite out of its orbit...
False. Despite the electromagnetic havoc they play with satellite electronics
and with Earth's magnetic field, the solar wind and CMEs barely exert a pressure or
force that you could measure. In fact, they couldn't ruffle the hair on your head.
The solar wind has fewer particles per cubic centimeter than the best vacuums
scientists have ever created on Earth. Our own air is billions of times denser
than the solar wind, such that a cubic centimeter of air has as many particles
as a cube of solar wind measuring 10 kilometers on each side.
Satellites do sometimes fall out of orbit due to space weather events. But
it is Earth's own atmosphere that drags them down, not pressure from the solar wind
or a CME. When a space weather event stirs up the space around Earth, the upper
atmosphere of Earth becomes hotter and denser, leading to more friction on the
surface of the satellite. This friction can slow a satellite down just enough
that it slowly - over months to years - can drop out of orbit.
- Auroras are caused by solar wind particles hitting Earth's atmosphere...
Not really. Auroras are caused by particles (mostly electrons) being guided
by Earth's magnetic field into the atmosphere, where they bounce off and collide
with air molecules. These collisions excite - give energy to - the air molecules.
To release that excess energy, the air molecules (mostly oxygen and nitrogen) emit
the light that we call the aurora. But the electrons that cause auroras do not
come directly from the Sun. At the beginning of the Space Age, scientists
discovered that the space around Earth is filled with plasmas - hydrogen
electrons and protons - trapped by our planet's magnetic field.
These particles are collected over long periods of time from the solar wind,
or they have leaked out of our own atmosphere. When a solar storm erupts,
the impact of the event can distort and energize Earth's magnetic field.
Some of this energy energizes the particles already trapped around Earth,
causing them to slide down those field lines into Earth's upper atmosphere,
to smash into the gas of the atmosphere, and to release photons of light.
- You can only see auroras (Northern and Southern Lights) at the poles.
False. Actually, auroras are quite rare at the geographic (or geomagnetic) poles.
As mentioned above, auroras occur when particles that are trapped inside Earth's
dipolar magnetic field slide down those field lines into the atmosphere. Auroras
occur (geographically) where these field lines connect down to Earth, principally
between 60 and 70 degrees of magnetic latitude (There are complicated, unusual
circumstances that can cause auroras at the actual poles, but those are not the
common forms of auroras that we see). The auroral zones of Earth take the form
of rings around the magnetic poles, most often cutting a path across Canada,
Alaska, Greenland, Scandinavia, and Russia (in the southern hemisphere, the
oval hangs principally over the ocean circling Antarctica). In fact, if
you made an expedition to the north coast of Alaska, you would likely have
to turn to the South to see the aurora. Learn more by visiting
http://www-spof.gsfc.nasa.gov/Education/waurora1.html
- Solar maximum is a single episode of high activity...or, the solar
maximum is a brief event that happens on a particular day...
False. Solar maximum describes the period - usually one to one-and-a-half years -
when the Sun is most active. Solar maximum is the peak or
hump on a mathematical curve of the number of sunspots seen
from Earth each day. This number can rise or fall from day to day,
month to month, but eventually the plot shows a period when the average
number of sunspots is higher than at any other time in the 11-year cycle.
In other words, the day with the most sunspots does not have to be the peak of
solar maximum, nor is it necessarily the month with the highest number.
(It is similar to charting high tide along the seashore. "High tide" is
not necessarily the moment of the absolutely highest wave, but the time
when the average of wave heights is higher than at any other time in a day.)
The time of solar maximum can be guessed, based on past history of the
sunspot plot. But it can only be determined with certainty many months
after it is over, when scientists see when the number of sunspots stopped increasing
and started steadily decreasing.
- The best time of year to see an aurora is the winter...
Yes and no. Because it is dark for much longer periods of each winter's day,
you have a longer night in which to see an aurora. And when skies are clear,
they are a little less murky (with less haze, water vapor, etc.) in the wintertime.
However, it is also more likely for skies to be cloud-covered in winter.
Weather and darkness aside, there is no physical reason why auroras should
be more common in winter than summer. Low levels of auroral activity occur
every day of the year in the northernmost (or southernmost) regions of Earth.
For reasons that space physicists cannot fully explain, the best auroral shows
occur most often around the times of the vernal (spring) and autumnal (fall) equinoxes.
Great auroras -- the once-in-ten years sort that come down to Texas or the Caribbean -- can
occur anytime the Sun decides to produce a great space weather event.
- Sunspots are dark and cool...
True and false. Again, this is a matter of relativity (not Einstein's kind,
the comparative kind). When compared to the 5500-degrees Celsius
in the photosphere (the visible surface of the Sun), sunspots are cooler -
a mere 3500 degrees Celsius. But that is still hot enough to melt most solids
from Earth. And as for being "dark" - sunspots are actually about as bright as
a full Moon, but when compared to the brilliance of the rest of the Sun, they
pale (or should we say darken?) in comparison.
Other gee-whiz facts to impress your friends...
- The Sun is an average star...
This depends on how you define "average." On the absolute scale of the size of
active stars from largest to smallest, our Sun is about medium mass and girth.
But when you consider that most of the universe is made up of dying, shrinking
dwarves, our Sun is actually larger, hotter, more massive, and brighter than the
vast majority of all stars.
- Comet tails always trail away from the Sun...
True. Whether a comet is inbound or outbound from the Sun, its tails will
always point away from the Sun. That's because the comet's tails - they have
both a dust tail and an ion tail - are formed when the pressure and heat of
sunlight vaporize pieces of these icy snowballs, and the electromagnetic
properties of solar wind strip ions from the chemical elements inside the comet.
- Solar sails are pushed by (or harness the power of) the solar wind...
False. Described by Arthur C. Clarke and other science fiction writers,
solar sails are actually a viable concept for propelling spacecraft across
the void of space. The science behind solar sailing is that photons of sunlight
exert a faint but detectable force that can be collected and used to push a large sail.
It would take a long time to get the spacecraft going, but once you start moving,
the acceleration could be almost endless.
For more fun with science misconceptions, check out these web sites:
Bad Astronomy
http://www.badastronomy.com/
Bad Science
http://www.ems.psu.edu/~fraser/BadScience.html
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