24 PART 1^ |^ EXPLORING THE SKY
orbit. Th ey continue moving westward away from the sun and
then move back crossing the far part of their orbit before they
move out east of the sun again. To fi nd one of these planets, you
need to look above the western horizon just after sunset or above
the eastern horizon just before sunrise. Venus is easier to locate
because it is brighter and because its larger orbit carries it higher
above the horizon than does Mercury’s (■ Figure 2-10). Mercury’s
orbit is so small that it can never get farther than 28° from the
sun. Consequently, it is hard to see against the sun’s glare and is
often hidden in the clouds and haze near the horizon.
By tradition, any planet visible in the evening sky is called
an evening star, even though planets are not stars. Similarly, any
planet visible in the sky shortly before sunrise is called a morning
star. Perhaps the most beautiful is Venus, which can become as
bright as magnitude −4.7. As Venus moves around its orbit, it
can dominate the western sky each evening for many weeks, but
eventually its orbit carries it back toward the sun, and it is lost in
the haze near the horizon. In a few weeks, it reappears in the
dawn sky, a brilliant morning star.winter from June 22 to September 22, and summer from
December 22 to March 20.
Now you can set your friends straight if they mention two of
the most Common Misconceptions about the seasons.
First, the seasons don’t occur because Earth moves closer to or far-
ther from the sun. If that were the cause, both of Earth’s hemispheres
would experience winter at the same time, and that’s not what hap-
pens. Earth’s orbit is nearly circular. Earth is only 1.7 percent closer
to the sun in January and 1.7 percent farther away in July, and that
small variation isn’t enough to cause the seasons. Rather, the seasons
arise because Earth’s axis is not perpendicular to its orbit.
Here’s a second Common Misconception: It is not
easier to stand a raw egg on end on the day of the vernal equinox.
Have you heard that one? Radio and TV personalities love to talk
about it, but it just isn’t true. It is one of the silliest misconceptions
in science. You can stand a raw egg on end any day of the year if
you have steady hands. (Hint: It helps to shake the egg really hard
to break the yoke inside so it can settle to the bottom.)
In ancient times, the cycle of the seasons, especially the sol-
stices and equinoxes, were celebrated with rituals and festivals.
Shakespeare’s play A Midsummer Night’s Dream describes the
enchantment of the summer solstice night. (In Shakespeare’s
time, the equinoxes and solstices were taken to mark the mid-
points of the seasons.) Many North American Indians marked
the summer solstice with ceremonies and dances. Early church
offi cials placed Christmas day in late December to coincide with
an earlier pagan celebration of the winter solstice.
The Motion of the Planets
Th e planets of our solar system produce no visible light of their
own; they are visible only by refl ected sunlight. Mercury, Venus,
Mars, Jupiter, and Saturn are all easily visible to the unaided eye
and look like stars, but Uranus is usually too faint to be seen, and
Neptune is never bright enough.
All of the planets of the solar system, including Earth, move
in nearly circular orbits around the sun. If you were looking
down on the solar system from the north celestial pole, you
would see the planets moving in the same counterclockwise
direction around their orbits, with the planets farthest from the
sun moving the slowest. Seen from Earth, the outer planets move
slowly eastward* along the ecliptic. In fact, the word planet
comes from the Greek word meaning “wanderer.” Mars moves
completely around the ecliptic in slightly less than 2 years, but
Saturn, being farther from the sun, takes nearly 30 years.
Mercury and Venus also stay near the ecliptic, but they move
diff erently from the other planets. Th ey have orbits inside Earth’s
orbit, and that means they are never seen far from the sun in the
sky. Observed from Earth, they move eastward away from the
sun and then back toward the sun, crossing the near part of their
SunSunMercuryMercuryVenusEclipticEclipticVenusSunset, looking westSunrise, looking east■ Figure 2-10
Mercury and Venus follow orbits that keep them near the sun, and they are vis-
ible only soon after sunset or before sunrise when the brilliance of the sun is
hidden below the horizon. Venus takes 584 days to move from the morning sky
*You will discover occasional exceptions to this eastward motion in Chapter 4. to the evening sky and back again, but Mercury zips around in only 116 days.