Astronomy - 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 the heavens.

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.

The Solar 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.

The 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.

Viewpoint from Earth, the Sun appears to be in front of (in) the constellation of Leo.

Conjunction 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 conjunction.

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.

Venus 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.

Understand 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.

Earth's Moon

The 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 orbits Earth.

Luna Phases

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.

Magnitude 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 of -26.72.

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).

Magnitude Scale 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.

Variable Stars

Stars on the chart with (var.) indicate variable stars. The brightness of variable stars can increase in magnitude for periods of months or years.

Star Spectral

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.

Top 20 Brightest Stars

# Common Name

Apparent Magnitude

Scientific Name
  Sun -26.72 Sun  
1 Sirius -1.46 a CMa Canis Major
2 Canopus -0.72 a Car Carina
3 Rigil Kentaurus -0.27 a Cen Centaurus
4 Arcturus -0.04 a Boo Bootes
5 Vega 0.03 a Lyr Lyra
6 Capella 0.08 a Aur Auriga
7 Rigel 0.12 b Ori Orion
8 Procyon 0.38 b CMi Canis Minor
9 Achernar 0.46 a Eri Eridanus
10 Betelgeuse 0.50 (var.) a Ori Orion
11 Hadar 0.61 (var.) b Cen Centaurus
12 Acrux 0.76 a Cru Crux
(Southern Cross)
13 Altair 0.77 a Aql Aquila
14 Aldebaran 0.85 (var.) a Tau Taurus
15 Antares 0.96 (var.) a Sco Scorpio
16 Spica 0.98 (var.) a Vir Virgo
17 Pollux 1.14 b Gem Gemini
18 Fomalhaut 1.16 a PsA Piscis Austrnus
19 Becrux 1.25 (var.) a Cru Crux
(Southern Cross)
20 Deneb 1.25 a Cyg Cygnus

The Planets of the Solar System


The 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.


Venus 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 -4.4


Earth, 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.


Mars 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


Jupiter 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 takes 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.


Saturn 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.



Uranus 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.



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.



Pluto 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.