Institute of Astronomy

Earth-like Moons part 2

Published on 21/11/2013 
Question: 

I am a writer and need more info on the question : Earth-like moons

Since the tides are higher would sailing be possible? Also, the seasons; would winter be longer on the dark side of the gas giant?

In and of themselves the much higher tides that one would expect on an Earth-like moon of a gas giant would not be a big problem for sailing.  There would be a lot more small islands, sandbanks, etc. that appear at low tide and disappear at high tide, so it would be necessary to be more careful around the coast and have accurate charts in order to avoid grounding your ship, but out in the middle of the ocean there wouldn't be any issues at all.

The biggest issue would probably not be any direct impact of the tides on the act of sailing, but rather on the ports and shore facilities that are necessary to support fleets of sailing vessels. As an example take what would be a relatively modest mid-ocean tidal range of 5m, roughly ten times that we experience today. Translating that directly into tidal ranges at the coast is not totally straight forward because geographic features can interact with the tide making the local range higher or lower.  For example in the Bristol channel the present day tidal range is up to 9m. Let's assume that here in Cambridgeshire and East Anglia the tidal range is about 10m.  In Cambridgeshire that would mean that at high tide the waters edge would be in the city centre in both Cambridge and Peterborough.  At low tide on the other hand the Wash would be high and dry and the sea would be well over 100km from Cambridge, even Skegness would several 10s of kilometres inland.  This is partly because of the Fens are low lying and the Wash is shallow, but even if we moved elsewhere the effects would still be significant.  If we went to Bristol for example even if we only increased the tidal range to 20m, a much smaller amplification than the Bristol channel experiences today, the Severn would be tidal all the way to Kidderminster and at high tide Cardiff, Newport and Weston-super-mare would all be under water.  At low tide there would be probably no more than 100 metres or so of water separating England and Wales as far down as Exmoor.

The issue of very large tidal plains could certainly be overcome, ships have always generally entered and left harbour at high tide, but it would make things more difficult.

As to the seasons there would not be any substantial effect (assuming that you maintain the same tilt of Earth's axis of rotation).  There would be regular eclipses whenever Earth passed behind the gas giant, but these would only be a few hours every month, not sufficient to have a dramatic effect on climate.

Page last updated: 21 November 2013 at 12:49