Leeds Astronomical Society LAS Meetings Observing Membership



Comets (& Asteroids)

Scroll & select a comet from the previews below to display the respective image(s).

  • C2020/F3 (Neowise)
    C2020/F3 (Neowise)
  • C2019/Y4 (Atlas)
    C2019/Y4 (Atlas)
  • Comet 17P (Holmes)
    Comet 17P (Holmes)
  • C/2006 M4 (Swan)
    C/2006 M4 (Swan)
  • C/2006 A1 (Pojanski)
    C/2006 A1 (Pojanski)
  • Comet 9P/Tempel 1
    Comet 9P/Tempel 1
  • C/2004 Q2 (Machholz)
    C/2004 Q2 (Machholz)
  • C/2004 Q1 (Tucker)
    C/2004 Q1 (Tucker)
  • Comet 78P (Gehrels)
    Comet 78P (Gehrels)
  • C/2003 K4 (Linear)
    C/2003 K4 (Linear)
  • C/2003 T4 (Linear)
    C/2003 T4 (Linear)
  • C/2002 T7 (Linear)
    C/2002 T7 (Linear)
  • C/2001 Q4 (Neat)
    C/2001 Q4 (Neat)
  • Asteroids Psyche & Parthenope
  • Asteroid (7482) 1994 PC1
    (7482) 1994 PC1

C/2004 Q2 (Machholz)

Near M45 - the Pleiades 08/01/2005
With M45 - the Pleiades 05/01/2005
Near M94 in Canes Venatici.

C/2004 Q2 (Machholz)

This comet, discovered in August 2004, is one of a dozen to have been found by American amateur astronomer Donald Machholz.

A Long-period comet with an undetermined period of the order of something like 12,000 years, it begame visible to the naked eye in Jan. 2005. Spectroscopic studies of it's volatile chemical composition indicate that it had previously visited the inner Solar system, where it's relative molecular abundance had been altered by heating from the Sun.

In 2005 the comet's perihelion (closest point) was > 1 AU (i.e. greater than the distance from the Earth to the Sun), which is unusual for a bright comet.

Judging by the number of pictures LAS members took, it must have created quite a buzz at the time!

For more info. see the Wikipedia entry.



Astronomical Units

The Astronomical Unit, or AU was originally defined as the average distance from the Earth to the Sun when at the closest and farthest points apart (called the aphelion and perihelion respectively). i.e. 1 × AU is about 150 million km (93 million miles) or about 8 light-minutes.

In 2012 the AU was defined to be exactly 149,597,870.7 km.

Measuring distances in Astronomical Units, provides a reasonable 'yardstick' for measuring distances on the scale of the Solar system, which would otherwise become unwieldy if they had to be represented in km or miles.

Stellarparallax parsec1

For the even larger distances outside of the Solar System, the Astronical Unit forms the basis of another distance measurement the parsec.

A parsec is defined as the distance which one AU subtends an angle of one arcsecond (1/3600th of a degree), and is equivalent to approx. 206,264.8 AU's or 3.262 light-years.

i.e. if when the Earth moves through a distance of 1 AU, a star appears to move by 1/3600th of a degree when compared to background stars, then the disance to the star is 1 parsec.

For example our closest major galaxy, Andromeda is 778,000 parsecs away, or approx. 160 billion AU (or 2.537 million light-years).



Apparent Magnitude

The apparent magnitude of a star is a measure of how bright it appears from Earth. The scale was introduced over 2,000 years ago by the Greek astronomer Hipparchus, who grouped stars into six categories. The brightest 20 or so were deemed to be 'first magnitude', slightly dimmer stars 'second magnitude', and so on until the barely visible stars were classed as 'sixth magnitude'.

Later it was recognised that our eyesight, once it has been given time to get used to darkness, has a logarithmic response. i.e. a Mag. 1 star is actually 2.512 times brighter than a Mag. 2 star, or 6.310 times brighter than a Mag. 3 star (2.512 x 2.512 = 6.310).

The six Magnitudes thus corresponds to a 2.5126 difference in brightness or 100x.

Apparent magnitude

Today the scale has now been extended, so that brighter objects can have an apparent magnitude of 0 or even negative. The brightest star Sirius, for example, has an apparent magnitude of -1.44 and the Sun is a whopping -26.74, due to it's close proximity to Earth.