Counting down to the winter solstice

FREIDL HALE
Last updated 13:56 31/05/2014
seven sisters, pleiades
DELWYN DICKEY

The Maori lunar year begins after the first New Moon that occures after the heliacal rising of Matariki, a cluster of stars known as the Seven Sisters or Pleiades.

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If you are out looking early enough - around 7.30pm at the beginning of the month - and if you have a clear sky, face north and you can see Jupiter, Mars, and Saturn scattered from west to east across our night sky. To help you spot them, on June 7 the Moon will be just above orange Mars, and on June 10 it will be almost touching Saturn.

If you have been following along you may have noted that Earth has passed by each of these three outer planets already this year, in our faster orbit closer to the Sun.

You can make a little demonstration on a table or the floor or in the yard, to help you visualise what is happening, and to see why Mars has been so bright and orange this year. You will need to clear the area except for one object in the middle to represent the Sun, a big ball is good but anything will do.

Next you will need two preferably smaller objects to represent Earth and Mars. If you want to be colour-coded maybe a red apple for Mars and a green apple for Earth. You'll need a measuring tape as well.

Place Earth about one metre from the Sun, and Mars a further half a metre away so that Earth is between Mars and the Sun.

Clearly Mars is one-and-a-half metres from the Sun and just half a metre from Earth. This is how we were positioned when we passed Mars on April 8.

Now move Earth around to the other side of the Sun, away from Mars, and place it again a metre from the Sun. You will see that when the Sun is between Earth and Mars their distance from each other is much greater. In fact in this position Mars is two-and-a- half metres from Earth, about five times farther away from us than it was when they were on the same side.

Of course, this is not precisely to scale, and in practice it is more complicated than that because Mars is also moving all the time and also because our orbits aren't perfect circles. It will be about two years and two months before we pass Mars again, but we do keep catching up and passing it and moving to the far side of the Sun from it, and then passing it again.

The same thing happens with each of the more distant outer planets. We periodically swing past them making a close approach. Then, as we continue on our way we get farther and farther away until we are on the far side of the Sun from them and then begin to approach again.

Venus, always the brightest planet, is brilliant in our eastern sky before sunrise, while Mercury is too close to the Sun to be easily seen, and will pass approximately halfway between Earth and the Sun on June 20.

The Moon will be full on June 13 and new on June 27.

June 28 is the first day of the month of Pipiri and the Maori new year. The Maori lunar year begins after the first New Moon that occurs after the heliacal rising of Matariki, an open cluster of more than 500 stars known also as the Pleiades or Seven Sisters.

Heliacal rising sounds complicated and requires an explanation. As Earth circles the Sun our night sky changes day to day as the stars and constellations appear to move across the sky throughout the year; they move from our night sky into the daytime sky and back around again.

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Matariki moves across our summer sky in the north, setting earlier and earlier in the west each night until it disappears into the sunset, which it did a few months ago. For a while it travels with the Sun across the daytime sky and then begins to rise before the Sun in the east.

There comes a day when it has moved far enough past the Sun to rise into a pre-dawn sky that is still dark enough for the dim group of stars to be briefly visible to the naked eye. This is known as "heliacal rising", heliacal from Helios, the ancient Greek god of the Sun.

While science can accurately predict the date when an object may first become visible in the pre-dawn sky, in practice it depends on actual observation, which can be difficult to say the least. Observation of a heliacal rising in different locations may be affected by many variables such as terrain, weather, or other atmospheric conditions.

The official heliacal rising of Matariki is on June 2, and, during the days that follow, it will become more easily observed as it rises higher in the dark pre-dawn sky. The first New Moon on which Matariki is visible in the pre-dawn sky is the beginning of the Maori new year. This will be the first New Moon after the heliacal rising, hence June 28.

This seems to me a good and fitting time for the start of the new year, especially since between these two events - the pre-dawn rising of Matariki and the new moon - the Sun reaches its northernmost point above the planet and reverses direction, returning south and bringing back its warmth and longer daylight hours. This event is known as winter solstice in the southern hemisphere and occurs on June 21. The harvests are finished and the land is at rest awaiting the burst of new life in the spring.

Note that in the ancient Aztec calendar the new year began with the heliacal rising of Matariki, and the new year in our standard calendar, developed in the northern hemisphere, begins on January 1, just a few days after the northern hemisphere winter solstice in December.

If you have a question or would like to receive or share information, please write to me at PO Box 152, Lake Tekapo 7945, or contact me by email: stars@tekapostarlight.co.nz

Clear skies at night,

Freidl Hale at Tekapo Starlight

- The Timaru Herald

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