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)
(+) 1035 How to calculate FOV for prime focus ?
(+) 1036 How do I calculate FOV for Eyepiece projection ?
(+) 1037 How do I use a Raspberry Pi camera for astrophotography ?
(+) 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 ?
What is an 'AU', a 'parsec' and a 'light year' (LY) ?
These are all measures of distance.
An 'AU' (or 'au', 'a.u.' or even 'ua', but never 'Au' which is the chemical symbol for Gold) is an 'Astronomical Unit'. One AU is the distance from the Earth to the Sun (about 93 million miles). First calculated by Astronomers in antiquity, it was a good 'shorthand' way of describing distances within the Solar System, especially when explaining 'Bode's Law' (see the next FAQ entry). One way to calculate the 'exact' distance was by observing the Transit of Venus (which occurs twice, 8 years apart, every 113 years - the last pair was 2004 / 2012 and the next pair will be 2117 / 2125)
These days it's seen by most as a rather quaint but obscure and antiquated term 'left over' from a time when Earth was considered to be the center of the universe (not that this stops some 'professional' Astronomers, who should know better, using 'AU' even today :-) )
Of course it was soon realised that the Earth's orbit around the Sun is ellipse - so the distance from Earth to the Sun varies from a maximum (aphelion) to a minimum (perihelion) and back again over each year. So 1 AU was redefined as the 'average' distance - however this means that the exact size of 1 AU changed every time a more accurate measurement of the Earths orbit was made ! Despite attempts to re-define the meaning of 'average', each time a new measurement was made, the size of 1 AU changed .. even Relativity played it's part when it was found that the definition was based on measurement techniques that did not allow for all relativistic effects, and thus 1 AU was not even the same for all observers !
It was only in 2012 when all the various definitions were finally dumped that the 'variability' of the AU came to an end. 1 AU is now precisely 149,597,870,700 metres 'by definition' (i.e. about 93 million miles :-) ). It is perhaps notable that 2012 was the year of the second (final) Transit of Venus this century )
Whilst the AU was a useful 'short hand' way of measuring distances within the Solar System it soon becomes cumbersome when applied to stars (for example, Proxima Centauri (the nearest star to Earth, excluding the Sun) is about 268,000 AU away and the distance to the center of our Galaxy (the Milky Way) is approximately 1,700,000,000 AU). So, to make life 'interesting', Astronomers (who were still trying to define what an AU was) invented the 'parsec' = which is approximately 3.26 light-years (and puts the center of the Milky Way at 8,330 pc).
Rather than try to explain what a Relative nonsense a 'parsec' turned out to be, I leave you with this diagram (right) and a link to Wikipedia
It was only with the Theory of Relativity that Astronomers finally realised that they already had a means of measurement that was exactly the same for all observers. Thus we have the Light Year (LY) - which is the distance travelled by light in one year (which is defined as 'exactly' 365 1/4 days, so no mucking about), and is exactly 9,460,730,472,580,800 metres (about 5,878,625,000 miles) - and this won't change as 1 meter is defined (since 1983) as the distance traveled by light in 1/299,792,458 of a second (thus the speed of light in vacuum is exactly 299 792 458 m/s). Since the speed of light is constant for all observers, astronomical distances no longer depended on where the Earth (and thus the observatory doing the measurement) happened to be in it's orbit 'relative to' the 'target' when the measurement was made.
To give you some scale, the Moon is about 1 1/4 light seconds away, it takes light 8 1/2 light minutes to reach the Earth from the Sun and from the Sun to Jupiter is 'only' about 43 minutes. Pluto is about 5 1/2 light hours away from the Sun = see here for a full list of the light distance to other planets.
To Proxima Centauri it's about 4 1/4 light years and the center of our galaxy, the Milky Way, is about 27,000 light years away.
This note last modified: 5th Feb 2015 22:05.
(+) 2115 What is Bodes Law ?
(+) 3010 When was Neptune discovered ?
(+) 4000 How do I update Stellarium with new Comet data ?
(+) 5000 How To build the MAS (Raspberry Pi) photoframe ?