2006 – The Major Lunar Standstill year
 

Have you been noticing the remarkable changes in the direction of the rising Moon as it goes through its phases, this year? This is an interesting year for such observations. Any given month, as one observes the Moon rise, one may notice that the direction where it rises from, changes. Sometimes it rises a little to the North of East, sometimes exactly due East and sometimes way south of East.

Every month, Moon makes these North-South movements. The Sun makes these movements once a year and we call this the Uttarayana and the Dakshinayana movement of the Sun. Sun rises exactly due East on the two equinox days, rises due North of East in Summer and due South of East during Winter. If we take the equator of the Earth and project it out on to the sky, we call this the celestial equator. The Sun crosses this celestial equator on the equinox days, is 23.5 degrees to the North of this on the day of the Summer Solstice and 23.5 degrees to the south of this on the Winter Solstice day.

Movements of the Moon are much faster, every month the Moon swings from North to South. However, in the case of the Moon, the maximum extent due North or South of the celestial equator that Moon is visible, changes.  Once in 18.6 years, these swings are at their maximum extent and 2006 is one of those years! In this year, the Moon can move 28.5 degrees to the North of the equator and swing all the way 28.5 degrees to the south of the equator all within a month. These extreme swings of the Moon once every 18.6 years are referred to, as Major Lunar standstills. Nine years later, the Moon will be swinging only 18.5 degrees due North or South of the equator.

All we need to do is find a reference point – a building or a tree due east from our house and observe the direction of Moon Rise with respect to this reference point. It will help if we use a compass, of course. One can even draw a circle on the ground and mark the directions on it. We can repeat the same observations about 9 years later and notice that, yes, the Moon seems to be swinging much less due North or South of East at rise times during that year, compared to its swings in 2006. This will be a Minor Lunar standstill year.

Why standstill? We need to look at the orbits of the Earth and the Moon a little, to understand this. Earth moves around the Sun in an elliptical orbit. The axis of its rotation is tilted to this orbit by 23.5 degrees. As a result of this, we see the position of the Sun in the sky changing with respect to the celestial equator (the projection of the Earth’s equator on to the sky). On the equinox days, the Sun shines overhead at the equator or we see the Sun on the celestial equator in the sky. During Summer it is to the North of this celestial equator, going 23.5 degrees to the north of this on the Summer Solstice day. In Winter it will move 23.5 degrees to the south of the celestial equator on the day of the winter Solstice. As the Sun moves towards the extreme points on the days of the Solstices, its Northward/Southward movement seems to come to a standstill and then reverses its direction. These are the Solar standstill points and in fact, that is the meaning of the word Solstice.

All this was an aside about the Sun. Coming back to the Moon, the orbit of the Moon around the Earth is tilted to Earth’s orbit around the Sun, by about 5 degrees. The location of the Moon in the sky moves around the celestial equator by an amount that is 5 degrees either added or subtracted from 23.5 degrees. During a Major Lunar standstill year, the Moon can swing by about 28.5 degrees to the North or the South of the celestial equator. Nine years later will be the Minor Lunar standstill year when it swings by 18.5 degrees to the North or South of the equator.

For cities like Delhi and other cities that are situated at a latitude of about 28.5 degrees, this adds a further interesting aspect to these Moon dances. From Delhi (or other locations situated at a latitude of 28.5 degrees) one can see the Moon move overhead on a few days, only once in 18.6 years. At other times one will not ever see the Moon move exactly overhead. For cities like Chandigarh, which are at a latitude of about 30 degrees North, the Moon will never be visible overhead, but, during the Major Lunar standstill year, it may be seen close to being overhead on some days.

A few of the days when Moon would be almost overhead from Delhi, in 2006, are the following –

The maximum northward swing of the Moon happens on the 15^th of September 2006, a day that Delhiites should watch out for, get up early in the morning and check out the Moon passing overhead, just at daybreak!

In contrast to these days are dates when the Moon swings southwards and hangs low in the south from rise to set. Even when it reaches its maximum height in the sky, it will appear low in the south. These dates for 2006 are the following :-

The date when the Moon makes the maximum swing southwards is the 13^th of June 2006.

In contrast to these values, one can compare with other years when Moon would not really climb overhead as seen from Delhi, until the next Major Lunar Standstill period 18.6 years later.

 Written by Dr. N. Rathnasree, Director, Nehru Planetarium, New Delhi.

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