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The moon.

Probably the most obvious feature of the Moon is that it passes through a number of phases from New Moon to Full Moon and back again (see images below). The angle from which the Sun illuminates the Moon is constantly changing, so the Moon goes through a cycle of phases of shadow every 29.53 days (synodic month). It is possible to have five lunar phases in a month whenever a lunar phase occurs on the first or second day. In February this is never possible since it has at most 29 days.

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With modest optical aid, many surface features are visible on the Moon. There are light-coloured highlands, interspersed with darker and less rugged lowland plains. The plains were, in the 17th century, believed to be seas, and ever since they have been called mare from the Latin for sea, or maria in the plural form.

The oldest and largest features on the Moon are the impact basins, huge multi-ringed crater structures, forming ridges and mountains, that are several hundred kilometres in diameter. There are more than 300 000 craters over 1 km in diameter on the Moon. The photographs on pp58-59 identify the most prominent features.

The line separating sunlight from shadow is called the terminator. It takes some two weeks to move across the Moon's visible surface. Through a telescope, movement of the terminator can be seen in a few hours by carefully studying mountain peaks and the insides of craters. More detail is visible near the terminator than anywhere else. Many craters turn into white spots, or disappear from view entirely, when lit from directly above, at Full Moon.

The Moon makes a full circle relative to the background stars every 27.32 days (sidereal month), moving its own apparent diameter (1/2[degrees]) in slightly over an hour. This motion is particularly obvious when the Moon occults a bright star or planet.

Lunar observers usually start out by learning their way around the Moon. Identifying the craters and other features shown in the photographs on pp58-59 would be a good exercise. The changing lunar phase can be followed by making sketches on successive days. First draw in the terminator, then sketch any prominent features and note how they change as the phase changes. More advanced observing programs include drawing individual craters, mountains, etc. at different solar lighting angles.

Eclipses

In 2009 a penumbral lunar eclipse occurs on August 06, the whole event being visible from Southern Africa. (Table 3a). A partial lunar eclipse occurs on December 31 and will be visible from the whole of Africa. (Table 3b). However first contact occurs as the Moon rises at sunset and will be difficult to observe.

A lunar eclipse occurs when the Moon passes into the shadow of the Earth, becoming dim until emerging from the shadow. The Earth's shadow consists of two parts - the dark inner umbra and the lighter outer penumbra. An eclipse does not occur every full moon because the Moon's orbit is orientated at more or less a constant 5.13[degrees] to the ecliptic and so an eclipse can only occur when the Moon is near to either of the two nodes of its orbit. The Moon does not always go completely into the umbra and this is called a partial eclipse. Penumbral eclipses are equally common but often go unnoticed because the darkening is so slight. A lunar eclipse is visible from anywhere on Earth where the Moon is in the sky at the time (unlike solar eclipses, which are only visible from within a much narrower path).

Total lunar eclipses last for up to 100 minutes, and do not require eye protection (unlike solar eclipses). During a total eclipse, the Moon darkens gradually as it moves through the penumbra, then more noticeably as it enters the umbra. The appearance of the Moon during totality depends on how much light is scattered by the Earth's atmosphere; the umbra is usually copper coloured, sometimes very dark, and occasionally distorted.

[FIGURE 1a OMITTED]

[FIGURE 1b OMITTED]

Lunar occultations

When the Moon passes in front of a star (or occasionally a planet), an occultation occurs. Stars always disappear at the eastern edge of the Moon (away from Mare Crisium) and reappear up to more than an hour later at the western edge. Stars disappear instantly; planets fade gradually over a few seconds. Double stars can disappear stepwise - first dimming as one component is covered, then vanishing completely a few seconds later. Several double stars have been discovered in this way.

The Moon's path through the zodiac results in a series of successive occultations of zodiacal stars. Within a series, there is an occultation at each conjunction of the Moon with the star, that is at intervals of 27.3 days. At the end of the series, the Moon will pass by the star and after nine years, a new series of occultations will take place. Each series has a duration of 17 months and contains about 20 events. The time interval between the middle of a series and that of the next is 9.3 years. The bright zodiacal stars that are currently undergoing a series of occultations are ? Tauri (first in series on 2005 Feb 16, last on 2010 Dec 19), [pi] Scorpii (2006 Feb 21 to 2009 Aug 27), [alpha] Scorpii (2005 Jan 27 to 2010 Feb 07), and s Sagittarii (2006 Dec 21 to 2009 Feb 20). Table 4 lists occultations of stars brighter than magnitude 6.5 that are visible from Southern Africa.

Grazing occultations

When a star moves tangentially to the limb of the Moon and is occulted for a very short period only, a few minutes, or even seconds, a grazing occultation is said to occur. Because the limb of the Moon, as seen from the Earth, is in fact the outline of numerous mountains and valleys, there may be several disappearances and reappearances, which are not only fascinating to observe, but which also may be accurately timed to yield valuable data on the relative positions of star and Moon, as well as on the shape of the Moon. Some of this data cannot readily be obtained in any other way. Details of grazes during 2009 are available from the Director of the ASSA Occultation Section.

Ramadan & Shawwal

Table 5 below gives the altitude and the difference in azimuth between the Sun and Moon at sunset for the period after New Moon on each occasion when the Moon is above the horizon. Positions of the Moon at altitudes less than 20[degrees] and differences of azimuth less than 15[degrees] are plotted on the diagram to the lower right.

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Sequence of phases

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New Moon; rises at dawn, sets at sunset; not visible; near the Sun; potential for a solar eclipse.

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Waxing crescent; rises after sunrise, sets before midnight; visible in western sky after sunset.

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First Quarter; rises at noon, sets midnight; 90[degrees] from Sun; visible late afternoon & evening; high at sunset.

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Waxing gibbous phase; rises after noon, sets before dawn; highest in the sky during late evening.

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Full Moon; rises sunset, sets at dawn; opposite Sun; visible all night; highest at midnight; eclipse possible.

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Waning gibbous phase; rises after the Sun sets and sets after dawn; high in the sky after midnight.

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Last Quarter; rises midnight, sets noon; 90[degrees] from Sun; visible 2nd half of night & early evening; high at dawn.

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Waning crescent; rises after midnight and sets after noon; visible in the eastern sky before sunrise.

African lore

* The Sotho-Tswana called the Moon Ngwedi, which is related to their word for month, kgwedi, showing their association of the moon with the monthly cycle.

* In African star lore lunation was used as a celestial analogy for the cycle of life in that the moon is seen to be born, to grow, to decay and die as it passes through its waxing and waning phases.

* In Zulu the description Izulu lihlanza inyanga "the sky is washing the Moon", is used to describe the rains at the time when the crescent Moon is expected.

* To the Shona and Venda the Moon was Mwedzi, or Nwedzi, meaning "the head of the kraal".
Quick facts about the Moon

Mass (kg) 7.349.1022
Equatorial diameter (km) 3.4762.103
Escape velocity (km.[s.sup.-1]) 2.38
Total mass of lunar atmosphere (kg) ~25 000
Surface pressure (bar) 3.10-15
Diurnal temperature range ([degrees]C) -170 to + 130
Distance from Earth (km) 3.63-4.06.105
Visual magnitude at opposition -12.74
Revolution period (days) 27.3217
Mean orbital velocity (km.[s.sup.-1]) 1.023
Recession rate from Earth (cm.yr-1) 3.8
Inclination to ecliptic ([degrees]) 5.145

Table 2. Lunar phases

New Moon Full Moon

Jan 26 09:55 Jan 11 05:27
Feb 25 03:55 Feb 09 16:49
Mar 26 18:06 Mar 11 04:38
Apr 25 05:23 Apr 09 16:56
May 24 14:11 May 09 06:01
Jun 22 21:36 Jun 07 20:12
Jul 22 04:36 Jul 07 11:22
Aug 20 12:02 Aug 06 02:55
Sep 18 20:44 Sep 04 18:03
Oct 18 07:33 Oct 04 08:10
Nov 16 21:14 Nov 02 21:14
Dec 16 14:02 Dec 02 09:30
 Dec 31 21:13

Table 3a. Penumbral lunar eclipse 2009 August 06

Event Date Time CT Bfn

(1) Contacts Aug 06 01:01:00 71.4[degrees] 72.9[degrees]
penumbra

(2) Maximum Aug 06 02:39:12 58.9 54.5
eclipse

(3) Leave Aug 06 04:17:24 39.8 33.6
penumbra

Event Jhb Dbn Har

(1) Contacts 73.8[degrees] 69.4[degrees] 74.4[degrees]
penumbra

(2) Maximum 53.6 50.3 51.3
eclipse

(3) Leave 32.0 29.4 28.5
penumbra

The last five columns give the lunar altitude for the various centres.
The penumbral magnitude of the eclipse is 0.428.

Table 3b. Partial lunar eclipse 2009 December 31

Event Date Time CT Bfn

(1) Contact Dec 31 19:15:05 -08.4[degrees] 00.20[degrees]
penumbra

(2) Contact Dec 31 20:51:26 08.3 16.4
numbra

(3) Maximum Dec 31 21:22:28 13.0 21.1
eclipse

(4) Leave Dec 31 21:53:37 17.4 25.4
umbra

(5) Leave Dec 31 23:29:54 27.9 34.7
penumbra

Event Jhb Dbn Har

(1) Contact 02.7[degrees] 03.1 [degrees] 09.1 [degrees]
penumbra

(2) Contact 19.5 18.9 27.2
numbra

(3) Maximum 24.3 23.3 32.4
eclipse

(4) Leave 28.7 27.2 37.1
umbra

(5) Leave 37.9 34.9 46.8
penumbra

The last five columns give the lunar altitude for the various centres.
The penumbral and umbral magnitude of the eclipse is 1.081 and 0.082
respectively. The total umbral time being 1 hour 2 minutes 11 seconds

Table 4. Lunar occultations of some bright stars

Date Star [m.sub.v] Place DTime

Jan 10 52 Geminorum 5.84 Jhb 22:56:31
Mar 03 TYC 6877-981 5.60 Bfn 02:48:06
Mar 16 SAO184258 6.06 CT
Mar 17 sigma Scorpii 2.92 Jhb 01:39:16
Mar 17 sigma Scorpii 2.92 Dbn 01:38:05
Mar 17 sigma Scorpii 2.92 Dbn
Mar 20 SAO187599 5.63 Bfn
Mar 20 TYC 6877-981 5.63 Dbn 02:53:18
Apr 01 SAO77837 6.04 CT 19:30:53
Apr 03 mu Cancri 5.30 Ptb 21:52:11
May 01 delta Cancri 3.94 CT 18:16:58
May 10 42 Librae 4.97 CT
Jui 09 rho Capricomi 4.86 Bfn 05:07:20
Jul 09 rho Capricomi 4.86 Dbn
Jul 18 TYC 1804-2519 5.30 Pta 05:19:06
Jul 18 TYC 1804-2519 5.30 Ptb 05:20:02
Jul 18 TYC 1804-2519 5.30 Jhb 05:31:11
Aug 02 lambda Sagittarii 2.80 Dbn 23:57:24
Sep 06 19 Piscium 4.95 CT
Sep 23 V913 Scorpii 5.43 CT 19:53:11
Sep 23 V913 Scorpii 5.43 Dbn 20:11:27
Sep 29 pi Capricorni 5.10 Dbn 00:00:00
Oct 07 18 Taurii 5.66 Dbn 21:53:13
Oct 11 omega Gem 5.20 Ptb 03:45:08
Oct 22 TYC 6830-20 6.40 Bfn 19:29:440

Date DCa[degrees] Phenomena RTime RCa[degrees]

Jan 10 -03[degrees]S Grazing
Mar 03 5.0[degrees]S Grazing
Mar 16 23:02:35 20[degrees]S
Mar 17 -20[degrees]N 01:53:46 01[degrees]N
Mar 17 -36[degrees]N
Mar 17 02:16:26 19[degrees]N
Mar 20 02:57:10 21 [degrees]S
Mar 20 4.2[degrees]S Grazing
Apr 01 12[degrees]N Conjunction
Apr 03 18.9[degrees]N Grazing
May 01 -15[degrees]S Conjunction
May 10 00:59:40 48[degrees]N
Jui 09 15[degrees]N Conjunction
Jul 09 05:22:34 38[degrees]N
Jul 18 13.6[degrees]N Grazing
Jul 18 13.7[degrees]N Grazing
Jul 18 -07[degrees]N 05:31:49 34[degrees]N
Aug 02 13.6[degrees]S Grazing
Sep 06 00:20:16 14[degrees]S
Sep 23 36[degrees]S 20:23:55 -12[degrees]S
Sep 23 35[degrees]S 20:35:19 -06[degrees]S
Sep 29 23[degrees]S 00:07:11 11 [degrees]S
Oct 07 04[degrees]N Conjunction
Oct 11 3.6[degrees]S Grazing
Oct 22 17.5[degrees]S Grazing

Key: Predictions for Cape Town (CT), Bloemfontein (Bfn), Durban
(Dbn), Johannesburg (Jhb),Pietersburg (Ptb), Pretoria (Pta) Type:
D = disappearance, R = reappearance. DCa: = disappearance cusp
angle, RCa: = reappearance cusp angle (a minus sign indicates a
bright limb event).

Table 5. Predictions for Ramadan and Shawwal

 Ramadan

Place Date Alt [DELTA]Az

Cape Town Aug 20 02.6[degrees] 1.7[degrees]
 Aug 21 16.0[degrees] [0.6[degrees].sub.1]
Johannesburg Aug 20 02.3[degrees] 2.0[degrees]
 Aug 21 15.7[degrees] [1.9[degrees].sub.3]
Durban Aug 20 02.0[degrees] 2.0[degrees]
 Aug 21 15.7[degrees] [0.8[degrees].sub.5]
Mecca Aug 21 08.2[degrees] [14.4[degrees].sub.7]

 Shawwal

Place Date Alt [DELTA]Az

Cape Town Sep 19 11.2[degrees] [2.6[degrees].sub.2]
Johannesburg Sep 19 10.3[degrees] [4.3[degrees].sub.4]
Durban Sep 19 10.4[degrees] [3.5[degrees].sub.6]
Mecca Sep 19 03.2[degrees] [11.4[degrees].sub.8]

First Quarter Moon

First quarter
lunar features

Craters

1 Piccolomini
2 Stevinus
3 Fracastorius
4 Theophilus
5 Langrenus *
6 Delambre
7 Macrobius
8 Posidonius
9 Atlas
10 Hercules
11 Burg
12 Eudoxus *
13 Aristoteles *
14 Aristillus
15 Manilius *
16 Julius Caesar
17 Horrocks
18 Hipparchus
19 Albategnius
20 Werner
21 Aliacensis
22 Stotler
23 Maurolycus

Mountains

TM Taurus
CM Caucasus

Other features

AV Alpine Valley
LM Lacus Mortis
LS Lacus
 Somniorum
MC Mare Crisium
MF Mare Frigoris
MFe Mare
 Fecunditatis
MN Mare Nectaris
MS Mare Serenitatis
MT Mare
 Tranquillitatis
MV Mare Vaporum
RV Rheita Valley

* These craters stand out well
during a lunar eclipse and can
be used to note the progress
of the umbra across the lunar
disk.

[ILLUSTRATION OMITTED]

Last Quarter Moon

Last Quarter
lunar features

Craters

1 Maginus
2 Clavius
3 Tycho *
4 Gassendi
5 Grimaldi *
6 Landsberg
7 Encke
8 Kepler *
9 Aristarchus
10 Bianchini
11 Pythagoras
12 Plato *
13 Aristillus
14 Autolycus
15 Archimedes
16 Timocharis *
17 Eratosthenes
18 Copernicus *
19 Herschel
20 Ptolemaeus
21 Albategnius
22 Alphonsus
23 Arzachel

Mountains

CM Carpathians
HM Harbinger
RM Riphaeus
SB Spitzbergen
StR Straight Range
SW Straight Wall
TM Teneriffe

Other features

MF Mare Frigoris
MH Mare Humorum
MI Mare Imbrium
MN Mare Nubium
OP Oceanus
 Procellarum
PE Palus
 Epidemiarum
SA Sinus Aestuum
SI Sinus Iridum
SR Sinus Roris

[ILLUSTRATION OMITTED]
COPYRIGHT 2009 Astronomical Society of Southern Africa
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2009 Gale, Cengage Learning. All rights reserved.

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Publication:Sky Guide Africa South
Geographic Code:60AFR
Date:Jan 1, 2009
Words:2687
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