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Astronomy / Astronautics 4.

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Answers Often Requested in news:uk.sci.astronomy, etc.

Topic FAQs

For general answers on observing, by unaided eye or by instrument, seek other sites.

There is a "FAQ for uk.sci.astronomy", by Stephen Tonkin, at the astunit site; it is particularly strong on amateur observation. Indeed, for most newsgroup matters see Stephen Tonkin's site.

There is a comprehensive, multi-part FAQ for news:sci.astro.

Newsgroup usage

Visual Observation

Meanings of Words

What does 'word' mean?  Try Chambers' Dictionary (printed; online), the OED, ST's Glossary, and Astro/Space Frequently Seen Acronyms.

Classification and Identification of Sightings

This assumes typical UK atmospheric conditions, and no optical aids.

For information needed when describing an observation, see also in Visual Observation; and read Majden on reporting (in list below).

If possible, give the object's position by reference to the stellar background; otherwise, by compass bearing and angular height (azimuth and altitude). Give the observing location, unless the object is certainly astronomical and located by background. Give also the time, and where appropriate the offset from GMT. Give the duration, if the object appears to be moving or changing.

Be reasonably accurate.

In addition, whenever it is not certain that the object seen is astronomical, try changing your position and see whether it moves correspondingly with respect to the stellar background. If it does, then it is nearby, and its distance can be estimated.

When travelling far, take a watch set to UTC/GMT, to avoid mistakes.

More-or-Less Astronomical Possibilities

Non-Astronomical Possibilities

Rough Limiting Naked Eye Magnitude Estimation

Data given by John Aldridge in News, based on the Yale Bright Star Catalog; but logarithms by me.

'Count' is cumulative, of all stars down to that magnitude; 'No/sr' is number per steradian, which is near to a comfortable eye-full.

Mag. -1.0-
Count   1  2  4  9 15224892 170285513892 160428475023
ln(C) 5.15.656.246.79 7.387.958.52
No/sr 13.522.740.871.0 127.6226.6399.7

Use of this may be a helpful preliminary before looking for known stars of known magnitudes, or deciding to look for a satellite.

The Angular Sizes of Things

Angular Size Seen

Partly because astronomical photographs and diagrams are frequently published without a clear indication of scale, it seems to me that many people are unaware of the actual angular sizes of objects as seen from Earth.

Object Span Seen from Earth
Moon 34.1'31.2'29.3'
Sun 32.5'31.9'31.4'
 Venus 61.5" 9.9"
 Mercury 11" 5"
 Mars 24" 3.5"
 Deimos Orbit 2.8' 24"
 Ceres 0.54" 0.25"
 Jupiter 47" 32"
 Europa Orbit 7' 5'
 Saturn 20" 16"
 Rings 44" 36"
 Titan Orbit 6.6' 5.3'
OriOrion's BeltAlnitak-Mintaka  2.5° 
UMaThe PloughAlkaid-Dubhe  26° 
AndNebula M31outer  ~3° 

1° = 60' ; 1' = 60" .

Geocentrically, objects in the first column circle Earth, and objects in later columns go round or belong to the object in the previous column. Check the figures; they are based on a trustworthy but approximate source.

Using the unaided eye, or spectacles if normally worn, and looking at a pair of bright "point" objects of similar magnitude, a young person with good eyesight can resolve ~?.?', and an experienced astronomer maybe ~?.?'.

A telescope with 75 mm (3") aperture can resolve about 2" of arc; a 200" (5 m) in principle about 0.03". Atmospheric 'seeing' limits actual achievement.

Angular Size and Magnitude

A change in angular size, for equal surface brightness, by a factor of 10 (one order of numerical magnitude) changes the received light by a factor of 100, which is 5 visibility Magnitudes.

The Moon, at 1 AU from the Sun and 1/400 AU from here, has a diameter of 3200 km and a maximum Magnitude of -12.6. Consider an object of similar material, at 1 AU from the Sun and 1/10 AU from here, with a diameter of 5 km. The angular diameter is less by a factor of 640×40 = 25600, so the Magnitude will be numerically increased by 5×log10(25600) for size, which is close to 22, making -12.6+22 = +9.4. There will be a further correction for being half-lit.

Those figures could, very roughly, be appropriate for Cruithne at closest approach; at the November 2006 approach, at 0.37 AU, it would have been around 3 Magnitudes dimmer.

Astronomical Errors

Common Errors

Some errors are reported as often taught, or as being in various training materials, or art/literature/books, or the press, in the UK or elsewhere. Any marked (n:uksa ...) have been mentioned in news:uk.sci.astronomy, at or about the indicated dates.

* But some now say that the warming is due to diversion of airflow by the Rocky Mountains.

Rare Errors

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