The ecliptic is the name given to
the path the Sun follows through the stars and constellations over the course of a year
.
We can’t see the stars behind the Sun because of the bright blue sky. The only exception to this is when the sky turns dark during a
total solar eclipse
.
The Wandering Sun, Moon, and Planets
The pattern of
stars
in the sky is fixed: the stars are so far away, they don’t appear to move relative to each other.
Against this fixed starry background, the
Sun
, the
Moon
, and the
planets
do appear to move, because they are much closer to Earth.
How close are the planets compared to the stars?
The nearest star to Earth is Proxima Centauri: it is 4.24 light years away. This means the light from Proxima Centauri takes about
4 years and 3 months
to reach us.
By contrast, light reflected from
Saturn
?the farthest of the planets visible with the naked eye?takes only about
1 hour and 15 minutes
to reach us.
The Path of the Sun
Although no stars are visible in the bright daytime sky, the names of the constellations the Sun passes through are well known (except one) as the
constellations of the
zodiac
:
- Pisces
- Aries
- Taurus
- Gemini
- Cancer
- Leo
- Virgo
- Libra
- Scorpius
- Ophiuchus
- Sagittarius
- Capricornus
- Aquarius
The
zodiac
is the name given to the strip of sky that runs along either side of the ecliptic. The zodiacal constellation that is not so well known is Ophiuchus.
Even though the Sun spends more time in Ophiuchus than it does in Scorpius, Ophiuchus was left out when the zodiac was divided into its 12 traditional ‘signs’ about 2500 years ago.
From Higan to Chuseok: Holidays connected to astronomic events
The Sun Does Not Travel Alone
The
solar system
is generally flat: the planets, and most of their major moons, orbit in more or less the same
plane
. (A plane is a flat surface, like a disk.)
This means that to us on
Earth
, the
Moon
and
planets
also tend to follow the
Sun
’s ecliptic path through the sky.
As a result, whenever we see them in the sky, the Sun, the Moon, and the planets roughly form a straight line. This line is the ecliptic.
How bright is the Sun on other planets?
The Ecliptic from an Outer-Space Perspective
Astronomers also explain the ecliptic from a different perspective, looking at the solar system from outer space.
The
apparent motion
of the Sun along the ecliptic is a result of Earth’s movement around the Sun. Another way to think about the ecliptic is, therefore, to say it is Earth’s
orbital plane
projected onto the
celestial sphere
.
Earth’s axis is tilted, so the
plane of the ecliptic
is inclined by about 23.4 degrees to the
celestial equator
. This angle is called the
obliquity of the ecliptic
, or
axial tilt
.
A Multi-Speed Solar System
Different celestial bodies move along the ecliptic at
different speeds
. The Sun takes one year to complete a full lap through the various constellations, but Saturn takes about 30 years.
The Moon, on the other hand, needs only 27.3 days to complete one full tour of the starry background. This is called a
sidereal month
.
The hidden patterns of Venus’s movement across the sky
Meetings in the Sky
The Sun, the Moon, and the planets often meet as they travel along the ecliptic. These events are called
conjunctions
. A famous example is the Great Conjunction of Jupiter and Saturn in
December 2020
.
A special kind of conjunction happens when the Sun and the Moon meet in the sky?the result is a
solar eclipse
.
“Ecliptic” Comes from “Eclipse”
Since the Moon passes by the Sun once a month, at New Moon, why don’t
solar eclipses
happen every month?
It’s because the Moon’s orbit is slightly tilted (by about 5 degrees) to Earth’s orbit around the Sun.
As a result, although the Moon generally follows the Sun’s ecliptic path, it is normally a little above or below it.
A solar eclipse can only take place when the New Moon crosses the ecliptic
.
In fact, this is the reason why the Sun’s path across the sky is called the ecliptic: it comes from the Greek word
ekliptikos
, meaning “of an eclipse.”
Topics:
Astronomy
,
Planets
,
Stars