LUNAR OCCULTATIONS

The occultations of stars by the Moon occur in series and each serie is separatede from the other by a period more o less long. Within a serie every occultation is separated from the following by an interval of 27.3 days. The orbital plane of the Moon is inclined respect to that of the Earth's orbit (the ecliptic) of 5░8.7' and the intersection of the two plane is the line of nodes; this line regress slowly in longitude respect to the stars of 19░21' per year and complete a rivolution in 18.6 years. In order that the Moon occults a star, it is no indispensable that the stars is exactly over the lunar node, but it can be in a range, infact the Moon isn't a dot and every serie of occultations lasts 17 months and includes about 20 events. The first occultation is visible from the southern polar regions, then the following from the equatorial regions and the lasts from the northern polar regions. The following serie will happen in reverse mode, from north to south. Because of the regression of the nodes every star will be occulted when it will be near the ascending or discending lunar node, therefore the interval between two series will be 9.3 years. The series of occultations and the periodicity depend from the ecliptical latitude of the stars: if they are less than ▒3░56ĺ they are 2 series of occultations in 18.6 years. If the stars are situated exactly on the ecliptic every serie lasts 1.4 years, if the are at ▒2░ it lasts 1.5 years, if they are at ▒3░ lasts 1.8 years. At 3░40ĺ lasts 2.2 years. For the stars with latitude from 3░56ĺ to 6░21ĺ, north or south, exist only one serie per period of 18.6 years (for example the Pleiades, Aldebaran, Antares). The most long duration is for the stars at ▒4░, almost 5.9 years, then the duration diminish and at ▒6░ last 2.2 years again. The more is great the latitude of a star and the more the zone of visibility of the occultations will be restrect around the equatorial zone of the Earth. The Moon can't occult the stars with latitude over 6░21ĺ (5░9ĺ+72.6ĺ media parallax). If ▀ is the geocentric latitude of the Moon, L is the ecliptical longitudine of a star, N the longitude of the ascending lunar node, p is the lunar parallax (p=57ĺ2.6" media) and s the medium lunar apparent semidiameter (s=15ĺ32.6") must be |B-▀|=1.21░, being 1.21░=p+s and B=5.145░sin(L-N). But since the lunar parallax and the distance Earth-Moon are variable, the value 72.6' varies actually between 68.6 and 78.3', therefore the Moon can occult stars up to latitude 6░36ĺ and not 6░21' as stated earlier. Here is a table with the 8 most brightest zodiacal stars interesting to observe. Very nice the Pleiades too.

 

Star

Name

Magnitude

L

a Tau

Aldebaran

1,1

69,789

-5,467

▀ Tau

El Nath

1,8

82,575

5,385

▀ Gem

Pollux

1,2

113,22

6,684

a Leo

Regulus

1,3

149,83

0,465

a Vir

Spica

1,2

203,84

-2,054

d Sco

Dschubba

2,5

242,57

-1,986

a Sco

Antares

1,2

249,76

-4,57

s Sgr

Nunki

2,1

282,39

-3,45

n Tau

Alcyone

(Pleiades)

3

59.992

4.051

Lunar geocentric occultations of bright stars by the Moon in this century

Lunar topocentric occultations of bright stars by the Moon in this century