The Sun is the central body of and main source of energy for the Solar System. With an equatorial diameter of 1,392,530 km (109 times the diameter of Earth) it is the largest body in the Solar System and holds approximately 99.85% of the Solar System mass. Composed predominately of hydrogen (~92% by number of atoms and ~ 75% by mass) with the remainder mainly helium, the Sun has a low density (1409 kg/m3) compared to the terrestrial planets (typically >5000 kg/m3). The thermonuclear fusion reaction that converts hydrogen to helium within the core of the Sun is the source of the energy that radiates out into the Solar System. The visible surface from which the Sun’s energy radiates is called the photosphere, it has a temperature of approximately 5500°C. Whilst of upmost importance to us on Earth, the Sun is considered to be an ordinary star with a spectral classification of G2 V, a yellow dwarf main sequence star.

The Astronomical Association Of Queensland Lunar Observing Programme commenced in August 2001 with the aims of:

• increasing the quantity and diversity of observational activities being undertaken by members; and to
• introduce planetary geology concepts that will provide members with an improved understanding of how lunar features observed through an amateur telescope formed.

The observing programme consists of 20 features that are easily observed through a small telescope and are archetypal examples of their kind. The programme includes a variety of craters of different morphology, mare, mountains, fault scarps, wrinkle ridges and rilles (valleys).

It is worthwhile reviewing the following basic facts on the Moon before commencing an introduction to lunar geology.

• The Moon orbits at a distance of 356,400 – 406,700 km from Earth with a sideral period of 27.32 days and a synodic period (the time between consecutive new moons) of 29.53 days.
• Tidal resonance between the Earth and the Moon has gravitationally locked the Moon into a captured or synchronous rotation (rotation period equals the orbital period) so that one side, referred to as the “near side”, always faces the Earth.
• The process of libration (lunar oscillations in longitude and latitude) brings 59% of the lunar surface into view from Earth.
• A low mean density of 3340 kg/m3 (compared to the Earth’s density of 5517 kg/m3) combined with a smaller diameter of 3,476 km results in the Moon having a surface gravity of one sixth of Earth’s gravity.
• The low surface gravity results in the Moon possessing a very tenuous atmosphere that is 14 orders of magnitude less dense than Earth’s atmosphere and contains only traces of hydrogen, helium, neon and argon.
• Two consequences of possessing negligible atmosphere are a large surface temperature range of approximately –230°C during the night to +120°C during the day, and the lunar surface is impacted by small meteoroids that would normally be destroyed in a denser atmosphere.
• There are two main terrain types are visible on the lunar near side (see Figure 1); rough, mountainous, intensely cratered, pale grey coloured highlands and relatively smooth, flat, less cratered, dark grey and often circular lowlands called maria or seas. In contrast the lunar far side is dominated by highlands with only a few small maria being present.

Comet 9P/Tempel 1 is due to be impacted by a projectile launched from NASA’s “Deep Impact” spacecraft at around 4.00pm AEST on 4 July. Although we won’t have dark skies at the time of the impact we will be able to observe it within several hours after impact (astronomical twilight ends at 6:30pm AEST). Some predictions estimate the comet may brighten from approximately magnitude 10 before the impact to 6th magnitude after impact. When observing the comet take note of any change in brightness or structure of the coma or tail. We recommend you observe the comet before impact (the Deep Sky Observing night at Mt Kent Observatory on the 2 July provides a good opportunity for this) and up to a week after impact. Finder charts to help you observe the comet and the aftermath of the impact are provided below. Charts have been provided below for the date of impact (Monday 4 July 2005) and for the Deep Sky Observing night on Saturday 2 July.

Refer to the NASA Deep Impact website and the Australian Sky & Telescope June 2005 magazine for more information on this exciting mission.

Click here for an all sky chart for 7pm on 2 July 2005.

Click here for a finder chart for Comet Tempel1 on 2 July 2005.

Click here for a finder chart for Comet Tempel1 on 4 July 2005. 

Click here for a detailed finder chart for Comet Tempel1 on 2 and 4 July 2005.