Celestial navigation and astronomy: Voyage from Newcastle to Sydney, day 3-4

Noon sights and calculating latitude

In clear conditions with minimal swell, day 3 of HMB Endeavour replica’s voyage was perfect for using sextants to measure the angle of the sun at its zenith.

Voyage crew and supernumeraries gathered in the waist of the ship at 1115 to practice using sextants. The sun’s highest point – known as Meridian Passage – would be at 1145, not noon, due to the time of year and the ship’s longitude.

Third mate Penny and supernumerary Bill measure the angle of the sun at its zenith. Photo: Eden Alley-Porter.

Third mate Penny and supernumerary Bill measure the angle of the sun at its zenith. Photo: Eden Alley-Porter.

One of our supernumeraries, Bill Morris, ran this session along with Penny, the third mate. Bill is an expert on sextants – he has collected 65 nautical sextants and is the author of The Nautical Sextant. His interest in navigation began at a young age.

‘I bought a book called Teach Yourself Navigation when I was about 14,’ Bill said. ‘I had no sextant and no horizon but I taught myself the rudiments of coastal navigation.’

Sextant. Photo: EAP.

Sextant. Photo: EAP.

The specific interest in sextants arose when he and his wife visited the Maritime Museum in London on their honeymoon. He didn’t really get to indulge this interest until he retired, many years later and then living in New Zealand. He has since started to collect and repair broken sextants.

Bill crossed the Pacific on a container ship, the Natalie Schulte last year – a 19-day passage from Auckland, NZ to Oakland, USA. During the voyage he took several sights on celestial bodies each day to determine the ship’s latitude.

After lunch on day 3, Bill and the other interested voyage crew and supernumeraries gathered in the Great Cabin to do the calculations required to determine the ship’s latitude.

Calculating latitude in Endeavour's Great Ccabin.

Calculating latitude in Endeavour‘s Great Ccabin. Photo: SMM.

There’s something very special about gathering in the Great Cabin to do this kind of work. Spangled reflections from the sea played across the bulkhead from the great windows in the stern as Penny explained the process of calculating the latitude.

At a table just like this one, in a Great Cabin just like that of the Endeavour replica’s Great Cabin, Cook would have followed similar calculations and would have worked on the detailed charts of Australia’s east coast.

IMG_2043

Calculating latitude. Photo: SMM.

At that time, there would have been little margin for error as there were no pre-existing charts to work from and no other way of working out the ship’s position.

Keen to repeat their successes of the previous day, some of the voyage crew gathered again on day 4 to measure the altitude of the sun – this time at 1146 and 44 seconds. The sun’s highest point is a little later each day as it moves north until the summer solstice on December 21.

With a little more swell running on day 4, Endeavour was not quite the stable platform that she had been on day 3. My calculated latitude was out by more than I care to admit, but Bill’s calculated latitude was within three miles of our GPS position.

Day 4 at sea

Overnight on Friday (day 3) the ship gained ground to the south under engines, taking us past Pittwater. With the strong southerly breeze, we were able to turn around and go sailing on Saturday morning – just for fun!

Furling the forecourse. Photo: EAP.

Furling the forecourse. Photo: EAP.

We began furling* sails around 1500 while sailing towards the entrance of Broken Bay. Furling continued once the rest of the sails were handed and we were waiting for Fred Watson AM and his partner and colleague Marni to arrive in the ship’s seaboat for the evening’s astronomy talk.

HMB Endeavour dropped anchor at 1700 in Broken Bay and we prepared for the last evening of the voyage.

Astronomy

The highlights of the evening were two sessions delivered by Fred about what we could see in the night sky. In the first, the sun had just set and Marni kept a keen eye out for the first star of the evening, Alpha Centauri (also known as Rigal Kentaurus), which is one of the two pointers to the Southern Cross.

Observing Saturn from the deck of Endeavour. Photo: EAP.

Observing Saturn from the deck of Endeavour. Photo: EAP.

Telescopes were set up to have a closer look at Saturn – thankfully there was no wind so the anchorage was very calm and the ship was a stable base for using a telescope.

After dinner, voyage crew returned to the deck with Fred to look at the after-dark night sky – including the Southern Cross, sitting low to the horizon at this time of year.

All’s well.

*Furling is the process of rolling or gathering a sail and securing it with gaskets (lengths of line) so that the sail does not flap in the wind.

– Suzannah Marshall Macbeth

Join the conversation...

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s