• The Cornwall & Plymouth Astrology Groups

  • Astronomy

  • Astronomy for Astrologers

  • Hi Everyone,

    As Astrologers we really need to understand enough Astronomy to underpin our knowledge of Astrology, so this is a brief introduction to the physical nature and the movements of the Sun and Moon and the planets of our Solar System that I hope will help you in this endeavour.

    The above image gives you an idea of the relative size of the planets.

    The relative distances can be considered by scaling the Sun-Neptune distance to 100 metres. The Sun would then be about 3cm in diameter and the giant planets would be all smaller than 3mm. The Earth's diameter along with that of the other terrestrial planets would be smaller than a flea (0.3mm) at this scale

    We’ll go through the details of each one now.

  • Sun

    The Sun is a nearly perfect sphere of hot plasma with a diameter of 1.39 million kilometres (109 times that of Earth), and has a mass 330,000 times that of Earth. This accounts for 99.86% of the total mass of the Solar System.

    The Sun is a G-type main sequence star (based on its spectral class) and is informally referred to as a yellow dwarf. It formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the centre, while the rest flattened into an orbiting disk that became the planets, moons, comets and asteroids of our Solar System.

    The Sun is roughly middle-aged, it has not changed dramatically for more than four billion years, and will remain fairly stable for more than another five billion years. At this point the core will experience an increase in density and temperature, the outer layers will expand and the Sun will grow to be a red giant engulfing Mercury and Venus and scorching the Earth.

  • Moon

    The Moon is the fifth-largest in the Solar System and the largest in relation to the size of the planet that it orbits.

    The Moon is thought to have formed about 4.51 billion years ago (not long after Earth) and the most widely accepted explanation is that the Moon formed from the debris left over after a giant impact between Earth and a Mars-sized body called Theia.

    Interestingly, when it first formed the Moon would have been 15 times closer to the Earth than it is now. Just imagine how it would have looked in the sky if there had been any one there to see it, and imagine the absolutely huge tides (maybe 300m high) racing around the planet!

    The Moon is in synchronous rotation with Earth which means it always shows the same face, with its near side marked by dark volcanic maria that fill the spaces between the bright ancient crustal highlands and the prominent impact craters. Its surface is actually quite dark with a reflectance only just slightly greater than that of worn asphalt, It’s the comparison to the dark night sky that makes it appears very bright.

    The Moon's average orbital distance at the present time is 384,402 km (238,856 miles) which is about thirty times the diameter of Earth. It’s apparent size in the sky is almost the same as that of the Sun resulting in the Moon virtually exactly covering the Sun during a total Solar eclipse.

    This is an amazingly precise co-incidence and you might think the Solar eclipse would be quite a magnet for passing Alien “tourists” out on a journey to see the wonders of the Universe!

    However this matching of apparent visual size will not continue in the far future because the Moon's distance from Earth is slowly increasing.

  • Mercury

    Because Mercury is much closer to the Sun than Earth it never appears more than 28 degrees away from the Sun.

    Mercury rotates in a way that is unique in the Solar System. It is gravitationally locked with the Sun in a 3:2 spin-orbit resonance. Which means that relative to the fixed stars it rotates on its axis exactly three times for every two revolutions it makes around the Sun. So as seen from the Sun it appears to rotate only once every two Mercurian years and an observer on Mercury would only experience one day every two years. 

    Mercury's axis is just about vertical (Earth’s is 23 degrees) and it’s orbital eccentricity (how oval it is) is the largest of all the currently known planets in the Solar System with the perihelion distance two-thirds of its distance at aphelion.

    Mercury's surface appears heavily cratered and is similar in appearance to the Moon's, indicating that it has been geologically inactive for billions of years. Having no atmosphere to retain heat, it has surface temperatures that vary from 100K (−173°C) at night to 700K (427°C) during the day across the equatorial regions.

  • Venus

    With it’s runaway greenhouse atmosphere causing surface temperatures of 735K (462°C) and a pressure 92 times that of Earth, some might wonder how it became know as Earth’s “sister” planet! However it does have a similar size and mass to that of Earth (though slightly smaller at 0.95 of Earth’s radius).

    Because Venus is closer to the Sun than Earth it never appears any more than 47.8 degrees from the Sun. It is the second-brightest natural object in the night sky after the Moon, reaching an apparent magnitude of −4.6 which is just about bright enough to cast shadows at night.

    Whereas Mercury has the most elliptical orbit in the known Solar system, Venus by contrast has the most circular. Also it has the longest axial rotation period (243 days) of any planet in the Solar System and rotates in the opposite direction to most other planets with an axial tilt only 2.64° from the vertical.

  • The pentagram of Venus is the path that Venus makes as observed from Earth. Successive inferior conjunctions of Venus/Sun repeat very near a 13:8 orbital resonance (Venus orbits 13 times for every 8 orbits of Earth), shifting 144° (2/5 of the circle) between conjunctions. (The resonance ratio of 8/13 is approximately 0.615385 while Venus orbits the Sun in 0.615187 years.)

  • Earth

    Home sweet home!

    All life is dependent upon the presence of water and our planet lies in the “Goldilocks” region of the Solar System where the temperature is not too hot and not too cold for liquid water to exist on the planet’s surface.

    However we are actually close to the inner edge of the Goldilocks zone and it won’t be long (in Geological time) before the Sun heats up enough to boil the oceans dry and sterilise the planet of multi-cellular life.

    In fact when the Greenhouse effect gets into full flow the Earth will become considerably hotter than Venus is now!

    Earth's axis of rotation is tilted by 23 degrees and it is this tilt that produces the seasonal variations. The gravitational interaction between the Earth and Moon causes ocean tides, stabilizes the Earth's orientation on its axis, and is gradually slowing its rotation. Earth is also the densest planet in the Solar System.

  • Mars

    Mars is often referred to as the "Red Planet" because the iron oxide prevalent on its surface gives it a reddish appearance that is distinctive among astronomical bodies visible to the naked eye. Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the valleys, deserts, and polar ice caps of Earth.

    The rotational period (24h 37m) and seasonal cycles of Mars are likewise similar to those of Earth, as is the tilt that produces the seasons (25.2° for Mars and 23.4° for Earth). However the orbit is more elliptical than the Earth’s varying between 128.4 and 154.8 million miles whereas the Earth varies between 91.4 and 94.5 million miles.

    Mars is the site of Olympus Mons, the largest volcano and second-highest known mountain in the Solar System, and of Valles Marineris, one of the largest canyons in the Solar System. The smooth Borealis basin in the northern hemisphere covers 40% of the planet and may be a giant impact feature.

    Mars is approximately half the diameter of Earth with a surface area only slightly less than the total area of Earth's dry land. It is less dense than Earth, having about 15% of Earth's volume and 11% of Earth's mass, resulting in about 38% of Earth's surface gravity.

  • Asteroids

    Of all the asteriods in the Asteroid belt between Mars and Jupiter, the only one large enough to form a nearly spherical “ball” is Ceres as pictured on the right here. The next largest is Vesta as pictured on the left. As you can see it is not quite large enough with enough gravity to form a “ball” (though maybe a rugby ball!) and for this reason it is not considered to be a dwarf planet under International Astronomical Union (IAU) Resolution XXVI.


  • To get an idea of relative size take at look at the image here of the 2 asteroids together with Pluto and it’s moon Charon.

    In fact if you add up the mass of all of the asteroids in the belt this would total about twice Charon’s mass (and significantly less than the mass of Pluto).

  • Ceres itself accounts for one third of the total mass of the belt and the four largest asteroids, Ceres, Vesta, Pallas and Hygiea together account for half of the belt’s mass.

    To put this into further perspective the total belt mass is just 4% of the mass of our Moon.

  • Jupiter

    A giant among the planets but still only one thousandth the mass of the Sun! Yet it is two and a half times the mass of all of the other planets put together!

    It has an elliptical orbit varying between 741 and 817 million miles from the Sun and rotates on it’s axis in slightly less than 10 hours which makes it the fastest rotating of all the planets.

    Orbital period is 11.86 years and the axis is nearly upright with an inclination of only 3.13°. It’s radius is 11 times that of Earth’s.

    As the first of the “Gas Giants” it differs significantly from the 4 inner “Terrestrial” planets in its size and composition. It is thought to consist of a dense core with a mixture of elements, a surrounding layer of liquid metallic hydrogen with some helium, and an outer layer predominantly of molecular hydrogen. Beyond this basic outline, there is still considerable uncertainty.

    The moons discovered by Galileo, Io, Europa, Ganymede, and Callisto are among the largest satellites in the Solar System with Ganymede having twice the mass of our moon. The eccentricity of their orbits causes regular flexing of the three inner moons' shapes, with Jupiter's gravity stretching them out as they approach it and allowing them to spring back to more spherical shapes as they swing away. This tidal flexing heats the moons' interiors by friction and almost certainly liquefies the ice into giant internal oceans which may provide some of the most promising habitats in our search for life in other parts of the Solar System.

  • Saturn

    Saturn is the other “Gas Giant” and has an average radius about nine times that of Earth but with a density much less than any other planet. In fact it is 30% less dense overall than water! So it would actually float if placed in a big enough ocean!

    This also means than were we able to walk at it’s surface, the apparent gravity would be less than that on Earth!

    Saturn has a similar shaped elliptical orbit to Jupiter, varying between 1.35 and 1.51 billion miles from the Sun, but unlike Jupiter it’s axis is tilted at an angle of 26.73° in a similar manner to Earth and Mars.

    It’s axial rotation time is slightly less than Jupiter’s at around 10.5 hrs and it takes Saturn 10,759 Earth days (or about ​29.5 yrs) to finish one revolution around the Sun. As a consequence, it forms a near perfect 5:2 motion resonance with Jupiter.

    Saturn has many moons, 53 of which have formal names. The largest is Titan, the only satellite in the Solar System with a major atmosphere in which a complex organic chemistry (with hydrocarbon lakes) occurs.

    Saturn is probably best known for the system of planetary rings that makes it visually unique The rings extend from 6,630 km to 120,700 km outward from Saturn's equator, average approximately 20 meters in thickness and are composed of 93% water ice with traces of tholin impurities and 7% amorphous carbon. The particles that make up the rings range in size from specks of dust up to 10m. While the other gas giants also have ring systems, Saturn's is the largest and most visible.

    There are two main hypotheses regarding the origin of the rings. One hypothesis is that the rings are remnants of a destroyed moon of Saturn. The second hypothesis is that the rings are left over from the original nebular material from which Saturn formed.

  • Uranus

    Uranus is similar in composition to Neptune, and both have different chemical composition from that of the larger gas giants Jupiter and Saturn. For this reason Uranus and Neptune are often classified as "ice giants" to distinguish them from the “gas giants”. Uranus's atmosphere is similar to Jupiter's and Saturn's in its primary composition of hydrogen and helium, but it contains more "ices" such as water, ammonia, and methane. It’s interior is mainly composed of ices and rock.

    Uranus is unique in the Solar System in that its axis of rotation is tilted sideways so its north and south poles lie where most other planets have their equators. (Its axial tilt is 97.77° to the orbit).

    Orbital time is 84.02 yrs and is slightly elliptic varying between 2.74 and 3.01 billion miles from the Sun. It has a rotation period of 17h 4m.

    The first of the “Outer Planets”, Uranus was discovered in 1781 by Sir William Herschel who initially thought it to be a comet. When discovered to be a planet Herschel decided to name it Georgium Sidus (George's Star) in honour of King George lll. Understandably however, Herschel's proposed name was not popular outside Britain! and alternatives were soon proposed. Astronomer Jérôme Lalande proposed that it be named Herschel in honour of its discoverer. Swedish astronomer Erik Prosperin proposed the name Neptune while in a March 1782 treatise, Bode proposed Uranus, the Latinised version of the Greek god of the sky, Ouranos. Bode argued that the name should follow mythology so as not to stand out as different from the other planets. He noted the elegance of the name in that just as Saturn was the father of Jupiter, the new planet should be named after the father of Saturn.

    Ultimately, Bode's suggestion became the most widely used, and became universal in 1850 when HM Nautical Almanac Office, the final holdout, switched from using Georgium Sidus to Uranus.

  • Neptune

    Neptune has 17 times the mass of Earth and is slightly more massive than its near twin Uranus, which has 15 times the mass of Earth. It is the densest of the giant planets and is actually smaller in size than Uranus even though it has more mass!

    Neptune takes 164.8 years to do a near circular orbit which only varies from 4.46 to 4.54 billion miles from the Sun. It has an axial tilt of 28.32° (which is similar to Saturn’s and only slightly greater than that of Mars and Earth) and it has a rotation period of 16h 6m.

    Some of the earliest recorded observations ever made through a telescope (Galileo's drawings on 28 December 1612 and 27 January 1613) contain plotted points that match up with what is now known to be the position of Neptune. On both occasions Galileo seems to have mistaken Neptune for a fixed star when it appeared in conjunction to Jupiter in the night sky, hence, he is not credited with Neptune's discovery. At his first observation in December 1612, Neptune was almost stationary in the sky because it had just turned retrograde that day. Because Neptune was only beginning its yearly retrograde cycle, the motion of the planet was far too slight to be detected with Galileo's small telescope.

    In 1821, Alexis Bouvard published astronomical tables of the orbit of Uranus. Subsequent deviations from these tables, lead Bouvard to hypothesise that an unknown body was perturbing it’s orbit.

    In 1843, John Couch Adams began work on the orbit of Uranus and produced several different estimates of the position of a possible new planet.

    In 1845–46, Urbain Le Verrier, independently of Adams, developed his own calculations and urged Berlin Observatory astronomer Johann Gottfried Galle to search with the observatory's refractor telescope.

    On the evening of 23 September 1846 Galle discovered Neptune within 1° of where Le Verrier had predicted it to be (about 12° from Adams' prediction).

  • Pluto

    Pluto was discovered by Clyde Tombaugh in 1930 and was originally considered to be the ninth planet from the Sun. After 1992, its status as a planet was questioned following the discovery of several objects of similar size orbiting further out than Neptune. In 2005, Eris, which is 27% more massive than Pluto was discovered. This led the International Astronomical Union (IAU) to define the term "planet" formally in 2006. That definition excluded Pluto and reclassified it as a dwarf planet.

    Pluto is currently the largest known “Trans-Neptunian” object by volume (though less massive than Eris) and is primarily made of ice and rock.

    It is relatively small at about one-sixth the mass of the Moon and one-third its volume and has an eccentric orbit during which it ranges from 30 to 49 astronomical units from the Sun (1AU = distance from Earth to the Sun). This means that Pluto periodically comes closer to the Sun than Neptune, but a stable orbital resonance with Neptune prevents them from colliding.

    Pluto's orbital period is presently about 248 years and it follows a path that is inclined to the ecliptic by over 17°.

    In the long term, Pluto's orbit is chaotic. Computer simulations can be used to predict its position for several million years but further than 10–20 million years, calculations become speculative.

    Pluto's rotation period is equal to 6.39 Earth days and like Uranus, Pluto rotates on its "side" in its orbital plane, with an axial tilt of 120°.

    Pluto’s large moon “Charon” (discovered in 1978) has half the diameter and one eighth the mass of Pluto.

  • Centaurs

    Centaurs are small solar system bodies orbiting between the outer planets. They have unstable orbits (with lifetimes of only a few million years) because they cross or have crossed the orbits of one or more of the giant planets. Centaurs typically behave with characteristics of both asteroids and comets and are named after the mythological centaurs that were a mixture of horse and human. It has been estimated that there are around 44,000 centaurs in the Solar System with diameters larger than 1km.

  • Chiron

    Discovered in 1977 by Charles Kowal, Chiron was the first identified member of the new class of objects now known as “Centaurs”. These are bodies orbiting elliptically between the asteroid belt and the “Kuiper belt” (a belt of asteroids/comets beyond the orbit of Neptune).

    The actual size of Chiron is presently uncertain within a diameter range of less than 200km up to possibly 271km. To put this in context, Pluto has a diameter 10 times greater at 2376km, while the Moon’s diameter is 3474km.

    Chiron takes 50.42 yrs to orbit the Sun and travels along an elliptical path taking it from the orbit of Uranus to just within the orbit of Saturn. It’s this orbital linking of Saturn and Uranus that is the “key” to the Astrological interpretation of Chiron. Also, unlike some other “Centaurs”, it’s orbit is only inclined at 7 degrees to the ecliptic thus linking it in more strongly to the orbits of the “Planets” which all lie very close to the ecliptic plane.

  • Pholus

    in 1992 the second centaur, “Pholus” was discovered. It has a diameter estimated to be between 165 - 190km and orbits the Sun once every 92 years at an inclination of 25° to the ecliptic.

    Although much more inclined to the ecliptic, it shares the “linking” orbital characteristic of Chiron. This time linking Saturn to Neptune. It does this by orbiting out to just past the distance of Neptune and then coming in to just less than the distance of Saturn. It’s this orbital contact that underpins the Astrological “quality” of Pholus.

  • Eris

    Eris appears to be entering into the collective human astrological consciousness so here’s a brief summary of it’s astronomy.

    Discovered in 2005, Eris was initially thought to be larger than Pluto, it was described as the "tenth planet" by NASA and in media reports of its discovery. In response to the uncertainty over its status, and because of ongoing debate over whether Pluto should be classified as a planet, the IAU delegated a group of astronomers to develop a sufficiently precise definition of the term planet to decide the issue. This was announced as the IAU's Definition of a Planet in the Solar System, adopted on August 24, 2006. At this time, both Eris and Pluto were classified as dwarf planets, a category distinct from the new definition of “Planet”. The IAU subsequently added Eris to its Minor Planet Catalogue, designating it (136199) Eris.

    Eris has an orbital period of 558 years with a maximum distance from the Sun of 97.65 AU (this occurred in 1977), and a closest distance of 37.91 AU. This brings it within the orbit of Pluto (30 – 49 AU) but not as far as Neptune (29.8 - 30.4 AU)

    Unlike the eight planets, whose orbits all lie roughly in the same plane as the Earth's, Eris's orbit is tilted at an angle of about 44 degrees to the ecliptic. (By contrast, Pluto’s inclination is 17 degrees).