- Observe the sky
Astronomy has an extensive
range of subjects. This page presents astronomy essentials, so
Astroplot charters and stargazers can observe and be aware of
Our Solar System has
a star at the centre called the Sun, which has celestial objects
gravitationally bound to it. Eight planets and three dwarf planets
orbit the Sun, including thousands of small bodies such as asteroids,
meteoroids, comets, icy particles and interplanetary dust.
The inner planets, in
order from the Sun, are Mercury, Venus, Earth and Mars. The four
inner planets have a rock composition. The four outer planets, Jupiter,
Saturn, Uranus and Neptune are gas giants located in a cold region.
A belt of asteroids orbit the Sun between the inner and outer planets.
The Kuiper belt of icy bodies is located at the outer region where
the dwarf planet Pluto orbits.
The first 6 planets from
the Sun are visible in the sky by the naked eye. Uranus and Neptune
can only be observed using a telescope. The further out in the Solar
System a planet is, the longer the distance and time it takes to
complete an orbit of the Sun.
System is vast, so it is depicted in two diagrams.
Above : The Solar
System planet sizes are compared.
Right : Planet appearances.
Our Solar System is
located in a galaxy called the Milkyway, which is comprised of
billions of stars.
Left : An illustration
of our Milkyway galaxy.
Right : From Earth,
the Milkyway, a band of compacted stars that look like a river
of cloud. The Milkyway, with Scorpio in the middle.
Solar System ecliptic backdrop
The ecliptic is the path
across the sky that the Sun travels during a year. For thousands
of years, people have been plotting the movement of the Sun, Moon
and planets. These celestial bodies move across the sky, through
patterns of stars, included in the Zodiac constellations. Time and
measurement was devised as astronomers plotted the location of the
Sun just after sunset or before sunrise. A celestial co-ordinate
system was devised based on the annual course of the Sun. The Sun
does not move, actually the Earth completes a 360 degree orbit around
the Sun in a year. The viewpoint from Earth of the stars is measured
to shift about 1 degree each day.
The Sun is the stationary
centre which the eight planets of the Solar System orbit.. Far out
in space, encircling the plane of the Solar system, are the stars
of the Zodiac. From Earth, the Sun appears to move along the ecliptic
across the Zodiac constellations. What moves during the year, is
Earth's viewpoint of the Sun against a background of stars.
All the planets orbit
the Sun in one direction, on a relatively even, similar plane. So
the planets are observed to move close to the Sun's ecliptic path.
from Earth, the Sun appears to be in front of (in) the constellation
and opposition of planets
Conjunction and opposition
have the same meaning in astronomy and astrology.
Viewed from Earth, a
conjunction occurs in the sky when two or more celestial bodies
appear close to each other. The term 6 degrees of separation applies
to the maximum distance between two bodies to be classified as a
conjunction. The lesser the degrees of separation, the closer the
Opposition occurs when an outer planet and Earth are the closest
to each other, and they are in direct line on the same side of the
Sun. Opposition only occurs with Earth and the outer planets. Opposition
is an ideal time to observe planets because they appear brighter
at their closest point to Earth. Also they are visible throughout
the night, rising around sunset and setting around sunrise. During
a period of opposition, a planet appears to be in retrograde.
and Mercury have superior and inferior conjunctions
Superior and inferior
conjunctions occur with Venus and Mercury, the two planets between
Earth and the Sun. An inferior conjunction occurs when one of these
planets lies in a direct line between Earth and the Sun. A superior
conjunction occurs when Earth is in line with a planet on the other
side of the Sun. During both types of conjunctions, Venus and Mercury
rise and set with the Sun. While moving on the eastern elongation
side, heading towards conjunction, these planets rise and set in
the morning. When located on the western elongation side, these
planets set after the Sun - the period when Venus is recognized
as the evening star.
Occultation occurs when
a celestial body such as the Moon moves in front of a star or planet
and obscures it. A planet can also occult a star.
Below : A plot of
the movement of Mars across Leo, over a 6 month period. Retrograde
began on the 12th of July when Mars was plotted moving backwards,
then on the 13th of October, Mars began to move forward.
Planets plotted against
the background of stars in Earth's sky normally move from west to
east (prograde). Planets are occasionally observed to slow to a
stop and move from east to a westward direction. Retrograde is proclaimed
when this occurs, when planets move in a reverse direction to their
normal direction. All the planets orbit the Sun in the same direction,
so retrograde observations are apparent, not actual. A planet in
retrograde is seen from Earth's point of view as we pass a planet
at an apposition alignment.
retrograde by considering a speeding car overtaking a slower car analogy.
You are driving in a racing car on a round track. As you approach
a car ahead, it seems to slow, then when you are side by side it seems
stationary, then it goes backwards as you speed onwards. A little
further, when you look sideways across the round track at the same
car, it looks like it is traveling at a regular speed.
Moon, or Luna, orbits the Earth, completing a circuit
once every 27.3 days. The distance between the Earth varies during
a Moon circuit, where its furthermost distance (apogee) is 406,697
km. The closest the Moon gets to Earth (perigee) is 356,410 km. The
Moon is in a synchronous rotation, so one face is always turned towards
Earth. The Moon has no atmosphere or erosion, so 30,000 meteor impacts
are visible to a telescope observer. The Moon is one-fourth the size
of Earth, with a diameter of 3,476 km, compared to Earth's equatorial
diameter of 12,756. The gravitational pull of the Moon effects the
tides on Earth. The full Moon can be as bright as magnitude -12.6.
Lunar phases occur
as the Sun illuminates the Moon at different angles as the Moon
The orbital period for
the Moon to complete a circuit around Earth is 27.3 days. The interval
of 29.5 days (the synodic period) between one full Moon to the next,
is due to the Earth and Moon orbiting the Sun together.
Every month, the Moon
passes across the 12 Zodiac constellations to complete a circuit.
The full Moon shines in the next Zodiac constellation as it did
the month before.
The full Moon is illuminated
by direct sunshine, when the Sun and Moon are on opposite sides
of the Earth. The Moon and Sun are virtually aligned, on the same
side of the Earth, during a new Moon, or dark Moon, because the
ray's of the Sun cannot shine on the side of the Moon that faces
Earth. An illuminated half Moon is due to only half of the Moon
facing the Sun.
There is a rise and set
time correlation with the Moon phases. A full
Moon rises as the Sun sets - the Moon is fully illuminated by a
direct angle of sunshine. A dark Moon sets in the west at the same
time as the Sun.
Waxing Moon: Increasing
phase. Illuminated surface of the Moon is increasing.
Waning Moon: Decreasing
phase. Illuminated surface of the Moon is decreasing.
Are you aware that people
in the Southern hemisphere view the Moon 'upside-down' compared
to Northern Hemisphere observers?
In the Southern hemisphere,
there is a way to tell if the Moon is waxing or waning. The Moon
is waxing if the right side is dark. The Moon is waning if the left
side of the Moon is dark. In the Northern hemisphere, the opposite
Moon observation occurs, where waxing occurs when the left side
is dark. The Moon is waning when the right side is dark.
is the apparent brightness measure of a star, planet or other celestial
body as observed from Earth.
Very bright objects have
a negative (-1) magnitudes. So, the brightest star seen in the sky,
Sirius, has an apparent magnitude of -1.46. The full Moon has an
apparent magnitude of -12.6 and the Sun has an apparent magnitude
The scientific names
for stars are relevant to the constellation that they are located
in. Based on the Greek alphabet, a (alpha) is the first brightest
star in the constellation. The second brightest is b (ß) (beta).
indicates the brightness of stars and planets.
The magnitude scale
under zero (0) begins at +1. The + sign is not usually indicated.
The brighter end of the + scale at (+1) to the faintest end of
the scale at magnitude (+12). Stars and planets below magnitude
+5, are so dim that the human eye cannot see them without a telescope.
A constellations is comprised
of stars located in an area of sky, as viewed from Earth. Astronomers
have named constellations according to the image they resemble,
by drawing imaginary lines to stars of to make pictures. Constellation
star patterns could depict pictures of mythic beasts or gods. The
constellation Leo, has a star formation that depicts a lion The
constellation of Libra is depicted as scales. Like landmarks, constellations
are sky-marks which are a references used to describe regions of
sky. There are 88 constellations.
Stars on the chart with
(var.) indicate variable stars. The brightness of variable stars
can increase in magnitude for periods of months or years.
Not all stars appear
white. There is a colour spectral scale for stars. Some stars have
a colour shade of blue, yellow or red. Two obvious red stars are
Antares in the constellation of Scorpio, and Betelgeuse, in the
constellation of Orion.
Below : The red star
Betelgeuse, in the Orion constellation.
20 Brightest Stars
Planets of the Solar System
planet closest to the Sun, Mercury, has no atmosphere. Always in the
region of the Sun, Mercury is not easy to locate as it rises and sets
during the twilight hours. The innermost planet takes 88 days to complete
an orbit of the Sun. Maximum brightness magnitude -1.9.
is nearly the same size as Earth. Venus is covered by thick clouds
of carbon dioxide that reflect the Sun to be the brightest planet
in the sky. Venus, also referred to as the 'evening star', can rise
up to 3 hours before sunrise, or set 3 hours after sunset. The period
Venus takes to circle orbit the Sun is 225 days. Venus has phases,
like the moon, which affects its brightness. Maximum brightness magnitude
the third planet from the Sun, is the largest of the inner planets,
with a surface area of 29% land and 71% water. Deep beneath the Earth's
rock crust is a thick molten mantle. At the centre of the Earth is
a metal core that generates a magnetic field. Earth is the only place
in the Universe known to have life that exists in an atmosphere of
78% nitrogen and 21 % oxygen. The Earth takes 365 days, a year, to
complete a circuit across the Zodiac and a rotation of the Sun. The
Moon orbits Earth.
has a reddish colour caused by iron oxide, rust, on its rocky surface.
Another feature is the ice caps at its poles. Half the size of Earth,
Mars is the last of the inner planets. Mars has two relatively small,
irregular shaped moons, Phobos and Deimos, which are thought to be
captured asteroids. A circuit of Mars around the Sun takes 687 days.
On average, Mars move across a constellation every two month. Mars
has a maximum magnitude of -2.8
is the largest planet in the solar system, being 2.5 times more massive
than all the planets combined. The planet has swirling clouds of ammonia
crystals and a great red spot which is an eternal storm. Jupiter is
composed of 10% helium and 90% hydrogen, with compressed metallic
hydrogen at its centre, surrounded by liquid and a gas atmosphere.
Jupiter has 16 moons, of which 4 (Ganyymede, Callisto, Io, Europa)
are large enough to see with binoculars as they orbit.
Jupiter is currently
in Gemini and will take 11.9 years to complete a circuit around
the Sun and across the Zodiac constellations. Jupiter takes about
a year to move across a Zodiac constellation to the next. The brightest
magnitude of Jupiter is -2.6.
has prominent ring formations made of ice, dust and rocks. The helium
and hydrogen composition of Saturn is similar to Jupiter. Saturn has
a many comparatively small Moons (satellites) among its rings, with
7 Moons large enough to be spheres, the largest Moon is Titan. Saturn
takes 29 and half years to complete a circuit of the Zodiac.
Saturn has commenced
a path across Libra. The maximum magnitude of Saturn is -0.3.
has a frozen hydrogen and helium centre, with an outer atmosphere
of methane which absorbs red light, so the planet has a cyan colour.
Uranus has 27 satellites, with its 5 main Moons in order from largest
are: Titania, Oberon, Umbriel , Ariel, Miranda.
Currently Uranus is in
Pisces, with a 84 year path to complete an orbit of the Sun. Uranus
is dim at +5.6 magnitude, so it is observed using binoculars or
Neptune is slightly more
massive than Uranus. The composition of the two planets is similar,
however Neptune's vivid blue colour and wind clouds are attributed
to more methane in its atmosphere. Neptune has 13 satellites, with
only one, Triton, larger enough to be a sphere.
Neptune is in Aquarius,
moving towards Aquarius. Neptune takes 165 years to complete a circuit
across the Zodiac during its orbit of the Sun. Look through a telescope
to see the planet with a brightness magnitude of +7.7.
was classified with Eris and Ceres as dwarf planets. The difference
between these bodies and planets are their comparatively small size.
Also, they have not cleared the 'neighbourhood' of their orbit. Each
dwarf planet is found in the 'neighbourhood' of a belt of orbiting
particle. Pluto, with its moon Charon, and another dwarf planet Eris,
are the largest bodies in the Kuiper belt. Ceres is located in the
asteroid belt between Mars and Jupiter. Pluto is in the constellation