Dr. Steve Bell

HMNAO/SSTD

The IoP/HMNAO MoonWatch Project

Islamic Astronomyat the Muslim World League

London September 8nd 2007

Dr. Steve BellH.M. Nautical Almanac Office

Dr. Steve Bell

HMNAO/UKHO

Visibility of the New Crescent Moon

37 hour-old Moon – ©James W. Young Table Mountain Observatory

Dr. Steve Bell

HMNAO/UKHO

Can you see the Moon?

Dr. Steve Bell

HMNAO/UKHO

Crescent Moon: November 14th 2004

< 50 hour old Moon

Dr. Steve Bell

HMNAO/UKHO

Moon Phases Moon Visibility1) Not visible2) Visible early evening, setting before midnight3) Visible first half of night, setting around midnight4) Visible most of the night, setting after midnight5) Visible all night6) Visible most of the night, rising before midnight 7) Visible last half of night, rising around midnight8) Visible early morning, rising after midnight

Dr. Steve Bell

HMNAO/UKHO

Observing the Crescent Moon

Moon diameter: 3476 km

Perigee: 356,500km (33.5’)Mean: 384,400km (31.1’)Apogee: 406,700km (29.4’)

Typical angular diameter 31.1’

The human eye can resolve ~1’ or a feature 110km across on the Moon’s surface. e.g. Tycho.

Crescent needs to be wider thanTycho to be seen by the human eye.

Dr. Steve Bell

HMNAO/UKHO

What do we need for sighting predictions?

• Calculate positions of the Sun & Moon

• Calculate rise/set times

• A knowledge of twilight

• Key quantities for calculating first sighting

• An algorithm to predict first sighting

• Generate global and local predictions

• Ability to improve the predictions

Dr. Steve Bell

HMNAO/UKHO

Visibility of the New Crescent Moon

Modern methods use relationship between the following quantities:• Arc of light (ARCL)• Arc of vision (ARCV)• Relative azimuth (DAZ)

Examples include:• Maunder (1911)• Bruin (1977)• Indian Astronomical Ephemeris (1996)

Dr. Steve Bell

HMNAO/UKHO

Historical Prediction criteria

• Earliest observations – Age > 24 hours in temperate latitudes

Equator: 16 – 25 hours

Latitude 30º: 17 – 33 hours• Babylonians – lag time > 48 minutes

Observations show a range of 22 – 84 minutes

• Medieval Islamic – lag time > 48 minutes with ARCL > 11.25º

Dr. Steve Bell

HMNAO/UKHO

Modern Prediction criteria

• Fotheringham (1910) ARCV > f(DAZ)

e.g. f(0º) = 12º, f(20º) = 10º • Maunder (1911) ARCV > f(DAZ)

e.g. f(0º) = 11º, f(20º) = 6º • Ilyas (1984) ARCV > f(DAZ)

e.g. f(0º) = 10º, f(20º) = 7º • Yallop (1980’s) ARCV > f(w crescent width) • Schaefer (1988) ?

Dr. Steve Bell

HMNAO/UKHO

HMNAO’s method

• Sunset time, Ts • Moonset time, Tm

-> Calculate Lag time = Tm - Ts

-> Calculate best time (Bruin’s visibility curves)

= Ts + 4/9 Lag time• Get topocentric altitude of Moon, am

• Get depression of Sun, ds

-> Calculate ARCV

= am + ds

Dr. Steve Bell

HMNAO/UKHO

HMNAO calculation: Yallop’s Criterion

q = 0.1 * (ARCV - 11.8371 – 6.3226 * w + 0.7319 * w2 – 0.1018 w3)

where:

ARCV is the sum of the altitude of the Moon and the depression of the Sun

w is the topocentric width of the crescent in arcminutes

Dr. Steve Bell

HMNAO/UKHO

Interpreting Yallop’s criterion

Crescent visibility:• A – easily visible q > 0.216• B – visible in perfect conditions q > -0.014• C – may need optical assistance q > -0.16• D – Will need optical assistance q > -0.232• E – Not visible with telescope q > -0.293• F – Beyond Danjon limit q <= -0.293

Dr. Steve Bell

HMNAO/UKHO

Danjon limit

• ARCL <= 8º

Why this value:• Rapid narrowing of the crescent near cusps• Illumination of the cusps by the Sun –

foreshortening relative to equatorial regions• Shadowing of the lunar surface – rough surface

illuminating at glancing angles• Earthshine comparable to crescent brightness

Dr. Steve Bell

HMNAO/UKHO

Global Visibility of the New Crescent Moon

We can generate a visibility criterion for a particular location using q.

What about a global view?

We can generate a grid of points and treat “q” as the basis of a contour map

What about the Ramadan 2007?

Dr. Steve Bell

HMNAO/UKHO

Making Sightings

• Record naked eye sighting: 15.4 hours• Record telescopic sighting: 12.1 hours

To make these sightings we need:-• Good atmospheric transparency• Clear weather conditions• Knowing where to look• Good visual acuity• Experience of observing

Dr. Steve Bell

HMNAO/UKHO

Moonwatch Background

• Most data for our models has been obtained at low to medium latitudes

• We need data from more extreme latitudes >50°

• Concentrate on non-record sightings

• We need careful observers to record sightings of the new crescent moon

Dr. Steve Bell

HMNAO/UKHO

The MoonWatch Project (I)

www.crescentmoonwatch.org

Dr. Steve Bell

HMNAO/UKHO

The MoonWatch Project (II)

Dr. Steve Bell

HMNAO/UKHO

The MoonWatch Project (III)

Dr. Steve Bell

HMNAO/UKHO

The MoonWatch Project (IV)

Dr. Steve Bell

HMNAO/UKHO

Crescent Moon Misidentifications

The new crescent moon can easily be confused with:

Cloud

Aircraft vapour trails

Other meteorological phenomena

Remember:

Repeat the observationto make sure

Dr. Steve Bell

HMNAO/UKHO

More Vapour Trails

Dr. Steve Bell

HMNAO/UKHO

2007 April 18th Sighting

31.8 hour-old Crescent Moon

Dr. Steve Bell

HMNAO/UKHO

2007 May 17th Sighting

25.1 hour-old Crescent Moon

Venus

Dr. Steve Bell

HMNAO/UKHO

MoonWatch Statistics

• More than 2700 attempted sightings have been logged

• 60% positive observations• 60% sightings from the UK• 60% positive sightings from the UK• Observations tend to come from a

hardcore of observers i.e. observe on 3 or 4 nights after new moon.

Dr. Steve Bell

HMNAO/UKHO

UK Distribution of Observers

Dr. Steve Bell

HMNAO/UKHO

Global Distribution of Observers

Dr. Steve Bell

HMNAO/UKHO

MoonWatch Preliminary Conclusions

• Almost all observations are consistent with the current model.

• More thorough analyses are required to look for second order effects

• Two potential record sightings have been logged

Dr. Steve Bell

HMNAO/UKHO

Possible U.K. Record sighting (I)

Sunset: 21:13 BSTMoonset: 22:28 BST

Best Time: 21:46 BSTSighting at: 21:30 BSTClassification: DElongation: 9°

Observer: First timeWeather: Western CloudsMoon Age: 15 hrs 04 min.

Observation logged on June 1st 2006 at 16:30 BST

Dr. Steve Bell

HMNAO/UKHO

Possible U.K. Record Sighting (II)

Sunset: 21:14 BSTMoonset: 23:38 BST

Best Time: 22:18 BSTSighting at:Classification: AElongation: 21°

Observer:Weather:Moon Age: 39 hrs 52 min.

Dr. Steve Bell

HMNAO/UKHO

Cochise County Arizona Sighting

Sunset: 19:18 MSTMoonset: 20:21 MST

Best Time: 19:46 MSTSighting at: 19:45 MSTClassification: AElongation: 12°

Observer: ExperiencedWeather: HazyMoon Age: 21 hrs 20 min.

Dr. Steve Bell

HMNAO/UKHO

Oxfordshire Ramadan Crescent Sighting

Dr. Steve Bell

HMNAO/UKHO

“Moonviz” Output for Birmingham

Dr. Steve Bell

HMNAO/UKHO

Ramadan 2007 (I)

Dr. Steve Bell

HMNAO/UKHO

Ramadan 2007 (II)

Dr. Steve Bell

HMNAO/UKHO

Ramadan 2007 (III)

Dr. Steve Bell

HMNAO/UKHO

Eid ul-Fitr 2007 (I)

Dr. Steve Bell

HMNAO/UKHO

Eid ul-Fitr 2007 (II)

Dr. Steve Bell

HMNAO/UKHO

Eid ul-Fitr 2007 (III)

Dr. Steve Bell

HMNAO/UKHO

A Final Request …

• A more thorough analysis is being undertaken

• More observations are needed

• Please keep on observing!

• Please submit your observations to www.crescentmoonwatch.org

Dr. Steve Bell

HMNAO/UKHO

Any Questions?

Dr. Steve Bell

HMNAO/UKHO

Sunrise/set times

• Need to know when the upper limb of the Sun is on the horizon or when the Sun has an altitude of -0 50’.

• We assume the horizon is at the same elevation as the observer.

• When the observer is H metres above the local horizon, the corrected altitude of the Sun is

– 50 – 2.12 √H

Dr. Steve Bell

HMNAO/UKHO

Rise/Set changes with height

La Palma – November 29th 2005

Sea-level (0m)

SR 7:46 SS 18:14

Mountain top (2326m)

SR 7:37 SS 18:22

Flying over La Palma

(10,000m)

SR 7:26 SS 18:34

Dr. Steve Bell

HMNAO/UKHO

Sunrise/set times for the UK

Dr. Steve Bell

HMNAO/UKHO

Sunrise/set times for the UK

Dr. Steve Bell

HMNAO/UKHO

Sun rise/set & twilight times in the UK

• Variation in sunrise/set times:

East-West 30 minutes

North-South 60 minutes

• Latitudes for the occurrence of twilight:

Astronomical < 48.5°

Nautical < 54.5°

Civil < 60.5°

Dr. Steve Bell

HMNAO/UKHO

Phenomena affecting sunrise/set times

• Parallax – only important due to the proximity of the Moon

• Refraction – dependent on meteorological conditions, adopted value of 34’ at the horizon

• Semi-diameter of Sun/Moon

• Beware – refraction affected by inversion layers, extreme meteorological conditions.

Dr. Steve Bell

HMNAO/UKHO

Accuracy of sunrise/set times

• Algorithm gives RA & Dec to 1 arcminute

• Good enough for rise /set times to 1-2 minutes

• Ultimately meteorological conditions place limit on accuracy of sunrise/set times.

Dr. Steve Bell

HMNAO/UKHO

Types of Twilights

Three types of twilight:-

• Civil: -0 50’ > Sun altitude ≥ -6• Nautical: -6 > Sun altitude ≥ -12• Astronomical: -12 > Sun altitude ≥ -18 • Assume 34’ for refraction at the horizon

and 16’ for the semi-diameter of the Sun.

Dr. Steve Bell

HMNAO/UKHO

What do they mean

• Civil: There is enough light to perform tasks requiring daylight, bright stars and planets become visible

• Nautical: The sea horizon is just visible

• Astronomical: Sixth magnitude stars can be seen at the zenith. Illumination from the sky is by starlight and airglow.

Dr. Steve Bell

HMNAO/UKHO

Post sunset sky

Dr. Steve Bell

HMNAO/UKHO

Twilight colours (I)

• 0º: orange, yellow, gray and blue stripes surrounding a blue-white arc reaching around 50º in altitude

• -2º: patch of pink light appears about 25º degrees above the horizon which dissipates and drops back towards the horizon

• -4º: orange-red, yellow, greenish horizontal stripes surrounding a salmon-red arc reaching around 25º in altitude. Blue gray above.

• -6º: orange-red, yellow and salmon-red stripes reaching about 10-15º. Purple-blue above.

Dr. Steve Bell

HMNAO/UKHO

Twilight colours (II)

• -9º: green-blue twilight glow up to altitude of 7-10º. Dark blue above.

• -12º: green–blue twilight glow around 6º in altitude. Dark blue above.

• -15º: green-blue twilight glow 3-4º high. Dark blue above.

• -17º: 5th magnitude stars visible, twilight glow has all but disappeared

• -18º: 6th magnitude stars visible, twilight glow has disappeared

Dr. Steve Bell

HMNAO/UKHO

“Belt of Venus” or Anti-twilight arc

← Earth’s shadow

← Belt of Venus

← Scattered sunlight

Dr. Steve Bell

HMNAO/UKHO

Almanac data for Rise/Set & Twilights

In the Nautical Almanac:

• Sunrise/set, Moonrise/set first introduced in 1925

• Astronomical Twilight – first introduced in 1928

• Civil twilight – first introduced in 1937

• Nautical twilight – as above

Dr. Steve Bell

HMNAO/UKHO

What about Dawn, Dusk etc?

• Dawn: Can mean morning twilight or sunrise

• Dusk: Can mean evening twilight

• Daybreak: Can mean twilight / sunrise

• First Light: Can mean astronomical or nautical twilight

Dr. Steve Bell

HMNAO/UKHO

Durations of twilight (minutes)

Date Equator

C N A

Mecca

C N A

London

C N A

Mar. 21

21 24 24 22 26 26 34 39 41

Jun. 21

22 25 26 25 29 31 48 74 ----

Sep. 21

21 24 24 22 26 26 33 39 42

Dec.21

23 26 27 24 28 27 40 43 41

Dr. Steve Bell

HMNAO/UKHO

Prayer Times

• Fajr – Beginning of morning astronomical or nautical twilight

• Zuhr – transit time of the Sun or after trailing limb of Sun has crossed the meridian

• Asar – length of shadow of stick = 1 or 2 x object length + noon shadow length

• Maghrib – Sunset• Isha – end of Civil / Nautical / Astronomical

Twilight or “reddishness” in dusk sky

Dr. Steve Bell

HMNAO/UKHO

Calculating Prayer Times

• Prayer times are a function of twilight times, shadow lengths or meridian passage

• At meridian passage, azimuth of Sun is 180°

Dr. Steve Bell

HMNAO/UKHO

Calculating prayer times

Shadow of a stick

Shadow length at transit: x = h / tan (a)

Shadow length at time t: xt = h + h / tan(at)

Need to find altitude at which satisfies

xt = x + h

i.e.

at = tan-1 (1/ (1+1/tan(a)))

Dr. Steve Bell

HMNAO/UKHO

Problems defining Prayer Times

• At higher latitudes, twilights may not occur

• Meteorological conditions can affect qualitative descriptions e.g. reddishness

Dr. Steve Bell

HMNAO/UKHO

Other options for prayer times

• Use times for location where phenomena occur.

• Split night into seven parts, Isha after first segment, Fajr after the sixth segment

• Use data from the day the phenomena last occurred

• Split the night into two.

Dr. Steve Bell

HMNAO/UKHO

Any Questions?