MAS Frequently Asked Questions
Frequently Asked Questions about astronomy (see also MAS Beginners page)
This page answers many commonly asked questions about astronomy. If you have a question on a subject any not covered here, please help support this page by using our on-line enquiry form (requires Java Script to be enabled).
Much of the material here has been taken from topics presented at out monthly meetings. If any member has subject they would like to present for 15-20 miniutes as a 'second half' topic, please contact the Meetings Secretary (to contribute directly to this page, please contact the Webmaster
(+) 0001 How do I find a local Astronomical Society ?
(+) 0002 How is Star brightness measured ?
(+) 0003 What are the Constellations ?
(+) 0004 What is the shape of the Earth ?
(+) 0005 What equipment do I need to start astronomy ?
(+) 0007 What telescope should I buy ?
(+) 0008 How much Magnification can I get ?
(+) 0013 What are Equatorial and Alt Az mounts ?
(+) 0100 How do I use my first telescope ?
(+) 0109 What is a finder ?
(+) 0110 What is an eyepiece ?
(+) 0111 What is a Barlow ?
(+) 0112 What is a focal reducer ?
(+) 0114 What are Nebular filters ?
(+) 0116 How do I use Setting Circles ?
(+) 0120 What is collimation ?
(+) 0125 How can I safely observe the Sun ?
(+) 1030 How can I take photos of the stars ?
(+) 1033 How to take photos of the Aurora ? - (Northern Lights)
(+) 1036 How do I calculate FOV for Eyepiece projection ?
(+) 1038 What is Star trailing ?
(+) 1039 How can I take photos of Meteors ?
(+) 2100 What is Universal Time (UTC) ?
(+) 2114 What are AUs Parsecs and Light Years ?
(-) 2115 What is Bodes Law ?
Bode’s Law and the discovery of the asteroids.
In 1772 a German astronomer, Johann Bode, drew attention to a curious mathmatical sequence which seemed to calculate the distance from the Sun to each to the 6 known planets. It implied that there was a missing planet between Mars and Jupiter. When a 7th planet (Uranus) was discovered outside the orbit of Saturn in 1781 and it was found that the new planet’s distance (19.2AU) agreed quite closely with Bode’s mathematical formula (19.6AU) this seemed to confirm there was indeed a 'missing' planet. However despite knowing the predicted distance (2.8AU) nothing had been found by 1790.
Taking a distance of 2.8AU, Astronomers set about predicting the rest of the 'missing' planet's orbital characteristics. They calculated that it would not approach Earth within 160 million miles, would move at a rate of one quarter a degree per day and be much fainter than Mars. This would imply a diameter of about 1000 miles (Earth is about 8,000, Mars about 4,000 and the Moon 2,000 so the 'predicted' planet would be half the size of our Moon) and, at 2.8 AU, it's size would be disc of 2” arc making it about 5th magnitude. Now there are several thousands stars between 5th and 7th magnitude, and all would have to be watched to notice any movement. The work was undertaken by a team of 24 astronomers organized by De Zach, and observations began toward the end of the 18th Century.
Each member of the team was given a particular portion of the sky to examine but no results would be expected for some time, because it would be necessary for each observer to become familiar with the star patterns (remember - this was 50 years before the invention of photography allowed the first photographs of the stars !).
Also at the end of the 18th Century, Piazzi was engaged in observing the heavens with the view of forming a great catalogue of stars, and while comparing a previous observer’s catalogue of Taurus with his own, he noticed that a star the previous observer had marked was no longer visible.
Surprised by the omission he carefully looked again in the same area to find it, but without success. On the fourth night of his search Jan 1, 1801, he saw a small star which he did not remember having noticed before. Then on the next night to his surprise the star seemed to have moved a little, so he carefully plotted its position. On Jan 3rd it had moved nearly 6’ arc from its previous location. He continued to watch the star and on the 10th night its movement became stationary, and then it moved in the opposite direction in longitude - which is exactly the observed behavior of all planets orbiting the Sun further out from Earth !
On Feb 11th 1801, illness prevented further observations so Piazzi reported his findings to the Italian astronomer Oriani and the German astronomer Bode. Unfortunately, by the time the letters reached their destination the object was too close to the Sun to allow further observations. Gauss used Piazzis' observations to calculate an orbit and predict it's position and when it would reappear on the other side of the sun, but when the search resumed the object could not be found. Harding, De Zach and Olbers were more persistent in their search and on the last night of the year 1801 De Zach detected 'a moving star' of about the seventh magnitude. Now that his discovery had been confirmed, Piazzi named new planet Ceres (after the Roman goddess of agriculture). It was found to move in an orbit whose mean distance was 2.77AU, while Bode’s law 'predicted' 2.80AU
Thus the first minor planet was discovered. The problem was that whilst the orbit was 'right', the size was not (7th mag. rather than 5th). Worse, other 'planets' were soon being found in the same orbit ! Olbers discovered Pallas three months later, and Harding discovered Juno on the 2nd of Sept. 1804. After observing for eight years, Olbers discovered Vesta (then in Virgo) at magnitude 6. This final small 'planet' convinced Olbers that all four where once part of the larger 'missing' planet (see footnote)
Bode's law was accepted as factual until the discovery of Neptune in 1846 (which, at 38.8AU, was too large a discrepancy from Bode's prediction of 30.1 AU to allow the theory to stand - however that's another story !)
Why does Bodes Law 'work' at all ? Well, it seems that a stable planetary system has planets that are 'regularly spaced' - this has been tested using the (mathmatical simulation of millions 'randomized' planetary systems which showed the ones with 'non-regular' spacing fell apart 'quite fast' when compared to the age of our Solar System (about 4.6 billion years)Ref : Astronomy by Edmond Neison FRAS (printed 1886)
Footnote: The assumption that the Asteroid Belt consists of the remains of a planet that 'exploded' billions of years ago remained unchallenged for hundreds of years (well, except by Lagrange, in 1814, who died the same year), thus providing an exciting 'plot line' for Science Fiction authors in the 20th century (even in 1977, in the classic Dr. Who series Image of the Fendahl it could be asserted that 'The Time Lords destroyed the 5th planet creating the Asteroid Belt' without expecting the viewers to burst into laughter). It was even taught in school 'Science Lessons' as recently as the mid 1960's (as some older members of MAS can confirm !)
However modern calculations show there is insufficient material in the belt to have ever 'made up' a complete planet (it's only about 4% of the mass of the Earth's Moon). The modern theory (which is more or less confirmed by space probes visiting Asteroids and discovering their composition) is that the Asteroid Belt consists of the icy rubble 'left over' during the formation of the Solar System and which ended up in this 'stable' orbit position. Since mathematical calculations show that Jupiter's gravity is too low to prevent the material accumulating, some other mechanism must have prevented the formation of a 'proper' planet at this orbit - unless, of course, the asteroids are the remnants of two small 'planetoids' that collided (see impact formation of Earth's Moon) - so perhaps the 'destruction theory' has some basis in fact after all !
** even today asteroids are being 'knocked out' of the asteroid belt into other orbits, some of which cross the Earth's and can impact the surface, although most are too small to do much damage (they mainly 'burn up' (and 'detonate') in the atmosphere). Even so, as recently as 65 million years ago, larger asteroids have proved that they are a threat to life on Earth
This note last modified: 5th Feb 2015 22:05.
(+) 3010 When was Neptune discovered ?
(+) 4000 How do I update Stellarium with new Comet data ?
(+) 5000 How To build the MAS (Raspberry Pi) photoframe ?