Stargazing at the End of Night

Sunset over Glen Docherty

This was me on the road and heading into the western Highlands last Saturday for my final stargazing gig of the season with the Woodland Trust.  

Skies this far north will shortly be too bright to stargaze with only Astronomical Twilight levels of darkness left near midnight and no official ‘night’ again until mid to late August. So do get out while you still can.  Of course the further south you are the less impacted you will be by this ‘near’ midnight Sun.

I had an eventful stargazing session with the Woodland Trust who were based at the Torridon for several nights. We first headed outside at about 10.30pm to view the crescent Moon with binoculars during early twilight skies – still too bright to see many stars apart from brilliant Arcturus.

The spring star Arcturus ‘Bear Watcher’ was bright enough to see in early twilight during our Moon gazing session.

After heading back inside for more projector based astronomy we ventured outside once more after 11pm and were fortunate to see a decent collection of bright stars and constellations despite some hazy cloud overhead.

Vega, Capella, Arcturus and Spica were all visible, in addition to the main stars in the Plough. I’d like to thank the Woodland Trust for inviting me and wish them well in their rewilding endeavours across the Highlands.

Spica and the Precession of the Equinoxes

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A line of the Moon, Jupiter and the bright star Spica can be seen tonight in the S/SW as the Sun sets (try after 11.30pm or later if it’s still too light).  Spica is the brightest star in the constellation Virgo and was the star that helped unearth the 26,000 year cycle known as the Precession of the Equinoxes.

As the story goes a temple in Thebes built around 3000 BC was originally aligned with Spica but the Heliacal sighting was found to have significantly drifted off target some 3200 years later.  This fact was taken up by the Greek astronomer, mathematician and geographer Hipparchus who used this and other data to approximate the precessional cycle time of the heavens (as he saw it) to be over 30,000 years.  An over estimate but a respectable first guess.

Of course we now know the real reason for this apparent cyclic drift of the constellations over long time scales – 25,772 years to be accurate.  A wobble in the earth’s local axis of rotation (see animation below) caused by the gravitational influence of the Moon and Sun on Earth’s equatorial bulge.

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There are a couple of interesting side effects of this dynamic.  Firstly our pole star slowly changes over thousands of years.  In ancient Egyptian times (3000 BC) the northern  pole star was Thuban in the Constellation Draco, and in around 12000 years time the pole star will have moved close to brilliant Vega.

The other effect is that the dates of the original Zodiacal star signs are now completely out of sync with the position of the Sun. A child born today (June 24th) is still designated as a ‘Cancer’ (because the Sun is supposed to be in the constellation Cancer right now).  But the Sun isn’t in Cancer – it’s actually in Taurus.