So a quick reminder, here we are with the Earth suspended inside the celestial
sphere, the celestial poles are just the projections of Earth's North and South
Poles, and the celestial equator is just a projection of Earth's equator. But over
the course of a year the Sun appears to follow a path moving eastward around the
sky and this path is called the "ecliptic". So I've Illustrated some key positions
of the Sun throughout the year and the month names represent where the Sun
appears to be with respect to the background stars at different times
during the year. So let's look at this from our vantage point on Earth. We're
taking an imaginary look at the Sun among the background stars. I've had to
turn off the atmosphere in order to allow us to see them and I've drawn the
ecliptic which is in orange, and remember this is the path that the Sun will
follow throughout the year. And I've centered the celestial equator as close
to the center of the screen as I could. And here we are on January 1st of 2018.
The Sun is in the constellation of Sagittarius. A couple of solar weeks
later the Sun has entered the constellation Capricorn, or Capricornus.
A couple of weeks after that the Sun enters Aquarius, followed by Pisces.
Now you notice that as the Sun is moving through these constellations, it is
climbing further and further north and you'll notice that by the 20th of March
it intersects with the equator so this position, this intersection between the
Sun coming from the south of the celestial equator and heading north is
called the "vernal equinox". Now a lot of people think that the vernal equinox is
a moment in time in fact it's actually just this particular point in the sky in
other words this is the moment where the Sun happens to be crossing from the
south to the north of the celestial equator
however the vernal equinox is always going to be there regardless of what day
it is so we often think of it as the first day of spring in the northern
hemisphere and for the folks in the southern hemisphere it's their first day
of autumn but it's really just a location in the sky and not an actual
date or moment. Let's allow the Sun to for buy a couple weeks, it now enters
the constellation Aries, followed by Taurus in mid-May and by the third week
in June it has entered the constellation of Gemini. At this point the Sun has
reached its highest position of the celestial equator and we have a name for
this position it's called the "summer solstice", but once again it's important
to remember that this just represents a location in the sky and not necessarily
a date. We're going to learn more about what it really means to define seasons a
little bit later on. Let's keep going and now in the third week in July we are
entering the constellation Cancer followed by Leo in August
followed by Virgo in September and a couple of days later on the 22nd we are
at what's called the "autumnal equinox". We're now transitioning from north of
the celestial equator to south of the celestial equator. A couple of weeks
later we enter Libra on the 31st of October followed by Sagittarius on the
23rd of November and Ophiuchus on the 30th of November. Now on this date of the 18th
of December we return to the constellation of Sagittarius, and on the
21st we reach our southernmost position beneath the celestial equator this is of
course the "winter solstice." And the Sun continues moving in its path until
returning to its starting position when it begins a new year. And the thing is is
that it's all about perspective. So in this example it's March and we are on
one side of the Sun but from our vantage point we see the Sun projected if you
will somewhere in the constellation of Pisces. Three months later the Sun has
drifted eastward at least it seems to our eyes it's drifted eastward but in
reality we have just simply continued in our orbit. Now the Sun is in Gemini
followed by Virgo in September, and in December it seems to have drifted
eastward again. And it isn't just the Sun that drifts eastward it
the sky from one day to the next but so do the moon and the planets. And you
notice that not only do they drift eastward, but they all seem to share
roughly the same path as the Sun does in other words they all seem to follow
along the ecliptic. So everything that we see here is traveling through these
highlighted constellations and these constellations may seem familiar to you
these are in fact the constellations of the zodiac. It's no surprise that ancient
astrologers, as they were called back in those days, thought that there might be
some special meaning to the fact that the Sun, the Moon, and the planets all
followed along the zodiac. It turns out that there is in fact a special meaning,
just not the one that you think of when you hear about astrology. What we really
are talking about is simply that all of the planets, including Earth, orbit the
Sun in the same plane. And we're going to learn more about the structure of the
solar system a little bit later on.