Home
About The Club
Club News
The Whirlpool Starparty
This Months Sky Guide
How to Join
Observing Sessions
Meetings Timetable
Gallery
Weekly Newspaper Article
The Committee
Links            
Messageboard

 

 

    
                              

 

                  

Newgrange -- The cycles of change  (Article No: 116.)

People have always recognised the importance of predicting 6he cycle of events in the heavens that shaped their lives. From time immemorial, the daily cycle of the Sun, the yearly cycle of the seasons and the monthly phases of the Moon were an intimate part of the culture and organisation of developed societies, great and small, throughout the world. As the growth cycle of plants is tied to the annual movement of the Sun, organised societies depending on agriculture tried to measure and predict the Sun’s cycle, to prepare for the sowing, harvesting and storing of food. An interesting local example of this is to be seen in Newgrange, Co. Meath. 5,000 years ago, a prosperous and highly organised agricultural community in this region, felt the need to measure the year’s length from one midwinter to the next and subdivide it into the seasons appropriate to sowing, tending, and harvesting of crops. When Newgrange was built, the architects had ensured that the rising Sun on the local horizon on Mid-Winter’s day would shine down the full length of the narrow passage that leads into the heart of the mound. Today, this doesn’t happen, which shows that there are long term changes going on in the celestial cycles that were undetected by these ancient astronomers.

Why doesn’t the Sun fully light up the back chamber of Newgrange? The answer is due to a slow nodding, or ‘nutation’ of the Earth’s axis from 24.5° to 21.1° and back again over a 41,000 year period. This means the tilt of the Earth’s axis can change by 3.4° over a 20,000 year period. We were all taught the inclination of the Earth’s axis is 23.5 degrees, which is true, for now. Since Newgrange was built, the position of sunrise at the solstice has moved by over half a degree due to this process, which is just over the angular size of the Sun’s disk. It may not seem a lot, but the passage marking the solstice sunrise 5,000 years ago was very accurately aligned, so even a movement this small would have a visible effect.

Another long-term cycle in the Earth’s rotation is called Precession. This refers to the circle in the sky traced by the Earth’s axis over a 25,800 year period. The obvious change it brings is the change in the pole star. Currently, Polaris is almost directly aligned above the North Pole. This will change in time as the axis continues its circular motion. 12,000 years from now, Vega, in constellation Lyra, will be the pole star.

Finally, the Sun itself constantly changes its apparent speed through the ecliptic throughout the year. This accounts for sundials being up to 8 minutes fast or slow compared to the more accurate measurements of modern timepieces. The elliptical shape of the Earths orbit means it travels at varying speeds around the Sun, which accounts for sundials being sometimes ahead, then behind time at various times of the year. Clear skies!

Visit www.shannonsideastronomy.com or call Conn on 061-301493.