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Astronomy / Astronautics 4.

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

• Standard news:uk.* conventions; for more on uk.*, see news:uk.answers and news:uk.net.news.*; also www.usenet.org.uk.
• The newsgroup Charter used to be posted to the newsgroup, monthly, sometimes; and is kept on the official UK News site; also, a copy is in the newsgroup FAQ.
• Do :- use plain text, fixed pitch, margin 72 characters; respond after each trimmed ">" quote.
• Don't :- respond to trolls, duplicate, over-quote, top-post, multi-post, HTML, multi-part, binary, pictures, ECP, EMP.
• Anonymous users get less respect.

Visual Observation

• What was that :-
  • In Classification and Identification of Sightings
• Which artificial satellites, etc., are currently/recently/soon visible, when and where :-
  • See at Heavens-Above; have your latitude and longitude in degrees and decimals to hand. There are rival sites; more links below.
• Which stars, planets, comets, etc., are currently visible, and where :-
  • Try local or national newspapers. On the first Monday of each month, the Daily Telegraph has a chart, alas seeming orthographic; the Times has a better monthly one, perhaps near-stereographic.
  • For stars, get a Planisphere (bookshop, etc.) or other form of star map.
  • See the weekly Sky & Telescope News Bulletin if still in news:sci.astro; and visit such as S&T or SkyMaps or The Salopian Web. Make allowances for latitude and sometimes for longitude.
  • Download and/or buy software such as SkyMap by Chris Marriott, Starry Nights from Garbo, etc.
  • Planet summary data at Heavens-Above with rise/transit/set times.
  • BBC monthly Sky Maps & Notes.
  • JavaScript Ephemeris - Peter Hayes.
  • Saturn's rings maximally visible, 2002; invisible, 2009.
• What else may be seen :-
  • AuroraWatch - U Lancs, UK.
• Where were you :-
  • For anything which is nearer than the Moon, or is unknown, reports of sightings should be accompanied by a sufficient indication of observer's location, two components of the direction, and the nature of any motion. Reports should also be accompanied by a sufficient indication of the time. UK civil time is GMT in winter and BST in summer, when the GMT day starts at 01:00 BST. See also Classification and Identification of Sightings
• Where am I :-
  • Latitude, Longitude, and Altitude can be determined from an Atlas; or from your local 1:50000 (or other scales) Ordnance Survey map, see the edges and the blue crosses. Using GPS is generally overkill.
  • UK Web Map sites show it - streetmap, Maporama, etc.
  • Perhaps use Autoroute or similar.
  • Position can be determined by visual observations of satellite times and comparison with predictions, with sufficient accuracy for most purposes.
  • Ask a Policeman.
• Where is (Geographic) North :-
  • The Sun is to the South half-way between sunrise and sunset (see the newspaper), at approximately 12:00 GMT and 13:00 BST in most of GB; a quarter-hour or later in Scotland and Wales; half-an-hour later in Ireland - see Equation of Time for variation.
  • The Full Moon is to the South 12 hours later.
  • See also Polaris, the Pole Star, via Ursa Major; it is about a degree from the Earth's axis. The true Axis direction is about 5% of the way from Polaris to Beta Ursae Minoris (Kochab).
  • More or less, the top of Orion's Belt is on the Celestial Equator, and his Sword points to the South Celestial Pole, 90° away.
  • Celestial North is on the Earth's rotational axis, about 23.4° from the Earth's orbital axis.
  • In Great Britain, Ordnance Survey maps (bookshops, libraries). give the relationship between Grid, True, and Magnetic North; and show the local direction of Grid North. And GPS can be used.
  • North on bodies other than Earth is a complex question with varying answers.
• What time is it now :-
  • The Heavens-Above Time Page site gives UTC, sidereal time, TLE time, TAI-UTC, MJD, JDN, etc., defaulting to German time. Login your details, uses your local time.
  • USNO gives UTC, and links.
  • BIPM gives UTC and TAI.
  • World Time Server gives UTC and local.
  • The BBC World Service Radio (648 m, etc.) gives GMT, and time pips.
  • See also my calculators for TLE, JD, MJD, CJD & CMJD.

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

• Objects substantially fixed in the Celestial Sphere :-
  • Permanent - constant/variable stars; a few nebulae; the Milky Way. Identify from star charts. Obs.
  • Temporary - novae, supernovae
• Objects varying in position on the Celestial Sphere :-
  • Planets - Star-like, or a little brighter; no obvious motion, but changing position with respect to the stars from night to night. See newspaper monthly charts. Obs.
  • Comets - Generally - just a small fuzzy patch of light; the better ones have obvious tails; motion as for planets. Obs.
• Objects visibly moving - Meteors - Generally, showing as a streak of light about as bright as a star, lasting for around a second; but can be much more.
  • IMO - The International Meteor Organization
  • Reporting Meteor Fireballs - Ed Majden (BC region)
  • Recent Fireball Sightings - SPA, UK
• Zodiacal Light, Gegenschein. Steady. From UK!?
• Aurorae - Extended varying coloured light structures, usually northwards. Obs.
  • Hourly STD DMSP/POLAR Auroral Activity Report
  • Welcome to AuroraWatch (UK)

Non-Astronomical Possibilities

• Artificial Satellites in LEO - Visibly moving if watched; about as bright as stars, possibly varying if rotating. ISS is very readily visible; and, when up, Shuttle & Tank are. Obs.
• Flares - Specular reflections of sunlight from satellite panels, can be much brighter than any star, Iridium up to Magnitude -8, rise and fall take several seconds, visible motion. Obs.
• Departures from LEO :-
  • Re-entries - moving a bit faster than a satellite, but will generally be breaking up.
  • Propulsive ?
• Atmospheric Phenomena :-
  • Weather - Causes optical effects. Sun-Dogs, Parhelia, Lightning, St Elmo's fire.
  • Irrelevancies - These include : aircraft and their lights, balloons (weather and other), kites, birds, fixed lights; fireworks; stray reflections.

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.

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.

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.

• The Time :-
  • At any instant, the civil time is not constant world-wide. So, in reporting the civil time of any observed event, the location must be given or obvious. Giving UTC, if known reliably, is better.
  • A positive Leap Second means that 23:59:60 UTC occurs; a negative one means that 23:59:59 UTC is omitted.
  • Event countdowns are often inefficiently written on astro websites - see "Good Countdowns" in JavaScript Date and Time 2 : Demonstrations. Also, offset designations such as EDT are not recognised by all browsers.
• The Date :-
  • At (almost?) any instant, the date is not constant world-wide. Therefore, in giving the civil date of any event, the location must be given or obvious.
  • Special cases :-
    • Solar Eclipses are always visible on only one civil date, except from near the Poles and when crossing the Date Line.
    • Lunar Eclipses are always visible on two civil dates; equally so, except for Time Offset asymmetry.
• The Seasons :-
  • The Earth's seasons are due to the tilt of the rotational axis with respect to the orbital axis, not to the shape of the orbit.
  • The Earth's perihelion (when the Earth is nearest to the Sun) is not in Northern Summer; it is currently around January 4th (which makes northern winters a little milder) (n:uksa 20040104). Also, the Gulf Stream warms Western Europe^*.
  • The Gregorian Calendar is closely (within 1 ppm) set to the Tropical Year and Vernal Equinox. Therefore, precession of the Earth's axis (period 26000 years) will not displace the Seasons in the Calendar; but it will change the perihelion effect.
  • Seasons, when defined by solstices and equinoxes, generally start on a later date in New Zealand than the opposite season does in Hawaii.
  • The Spring Equinox is in Spring everywhere, but the Vernal Equinox is always in March.
• The Earth :-
  • NASA's satellite-observation sites betray peculiar views on British places, their locations and their names.
• The Moon (and Planets) :-
  • The Moon has no permanent Dark Side (though it has a permanent Far Side [with a little wobble] as seen from Earth). And Mercury is now known to rotate with respect to the Sun.
  • Artwork may show stars within the unlit portion of the Moon, which is impossible.
  • Lunar Phases are not caused by the shadow of the Earth, but by the Moon moving round the Earth and being seen from different angles with respect to the illumination.
  • Given Phases may be incompatible with the position and the time. The illuminated part always faces the Sun.
  • Polaris is only the Pole Star for Earth; the rotational axes of different bodies (incliding the Moon) point in different directions.
  • Seen from the Moon, an eclipse of the Sun cannot be anything like annular.
  • The Ecclesiastical Full Moon, and so the Paschal Full Moon, are not equivalent to the Astronomical Full Moon. See The Date of Easter Sunday, ff.
• The Solar System :-
  • Analogies for the scale of the System may be wrong, at least for Earth orbit radius / Sun diameter.
• Spaceflight :-
  • Timings of launches directly to other planets are not governed by the timing of closest approach (opposition). They are chosen so that a near-Hohmann orbit reaches a point on the destination orbit at a time when the destination planet also reaches that point.
• General :-
  • The Lagrange Points are (obviously) not locations where gravitational forces are balanced. They are points where the combined gravitational forces provide the necessary centripetal force needed to maintain a particle's distance from the centre of rotation.
  • Temperature in degrees C or F is not proportional to its numerical value; but it is proportional in degrees Kelvin. So "twice as hot" only has physical meaning when using degrees Kelvin.

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

Rare Errors

• Daily Telegraph, Mon 2005-11-07 p.20 : Adrian Berry appears to believe that the Iridium 11 satellite is permanently and brightly visible, stationary near Beta Aurigae.
• Daily Telegraph, Mon 2006-02-06 : Adrian Berry apparently expects a New Moon on the 29th of the month.
• Daily Telegraph, Sat 2006-10-28 p.1 : "Remember to put your clocks back one hour at 1am tomorrow" - figure shows 01:00→00:00. But Sunday Telegraph, 2006-10-29 p.1 : "Reminder! Don't forget the clocks went back at 2am" - figure shows 02:00 - correctly.
• Daily Telegraph, Mon 2006-12-04 p.24 : Adrian Berry asserts : "The Sun uses up four million tons of hydrogen fuel every second.".
• Daily Telegraph, Mon 2007-02-05 p.24 : Camelopardalis is the Latin for Giraffe.
• Daily Telegraph, Mon 2007-03-05 p.22 : Leo is not, and can never be, directly overhead from the UK.
• Daily Telegraph, Mon 2007-06-04 p.24 : Hayabusa may return in June 2010, not June 2007.
• Daily Telegraph, Fri 2007-08-31 p.11 : Apophis diameter is about 240 metres, not miles.
• Wikipedia : ESA's Rosetta spacecraft, approaching for its second Earth fly-by on 2007-11-13, was identified by MPC as NEO 2007 VN84.
• Daily Telegraph, Mon 2008-03-03 p.26 : Adrian Berry asserts : "The Sun ... every second in its core, four million tons of hydrogen are converted onto helium". That is low by a factor in excess of 100.
• Daily Telegraph, Thu 2008-03-20 p.3 : Re "... Easter ..." by Petrie and Allen : "Why so soon?" is slipshod. The actual Equinox does not matter, March 21 is used. The Moon is not the real one, but an ecclesiastical approximation. The Full Moon may be on the nominal equinox. See in the Book of Common Prayer.
• Daily Telegraph, Mon 2008-05-05 p.22 : Adrian Berry asserts that the precession of the Celestial Pole is "due to eccentricities in Earth's orbit".
• Sunday Telegraph, Sun 2009-04-26 p.14 : Richard Gray, Science Correspondent asserts that ESA's Solar Orbiter "will orbit at around 20 million miles" (OK) and "will circle the Sun every 150 years". The period of Neptune is only a little greater than that; the period of Mercury is 88 days. The true figure is 150 days, for an elliptical orbit : ESA, Solar Orbiter overview
• The Times, 2009-09-11 : Chris Smyth : "These, known as Lagrange points, exist at the place where the forces of attraction from each of the two bodies are exactly equal." The Times should know better than that.