Cruising and Navigation Association of New Zealand


Welcome to the vernal equinox (last Sunday). That was when the overhead sun shifted from the northern hemisphere to the southern hemisphere. It is only on the equinox that the sun rises due east and sets due west.

In the diagram the word DECLINATION is used to describe the latitude where the sun appears to be directly overhead. This varies in a regular way throughout the year and can be taken as a marker of our seasons.


Astronomers, logically, time the seasons using the solstices and equinoxes of earth’s orbit around the sun. By this reckoning, spring starts with vernal equinox. Note that this makes our winter last 93 days and summer 89 days because the earth’s orbit is slightly elliptical and the earth speeds up a bit during our summer.

For practical purposes climatologists measure climate data averaged over each calendar month, and by this reckoning the months of September to November are taken as spring. Note that by this reckoning summer lasts 90 days (91 days in a leap year). In New Zealand this method of marking the seasons is more popular than using the equinox and solstice. Fair enough, it adds 1 or 2 days to summer Smile

You may have noted that the time between sunrise and sunset is NOT 12 hours on the day of the equinox, but around 7 minutes LONGER. This is because sunrise is defined at the time the top limb of the sun is just visible on the horizon, and similarly sunset is defined with the top limb of the sun disappears below the horizon. Due to refraction of the atmosphere the sun appears and disappears when is slightly below the horizon. This explains the “extra” day light. The name used to describe the date when time between sunrise and sunset reaches twelve hours is the equilux as described at

Over the next few weeks, as we get to notice the longer days there will be extra warmth reaching the southern ocean. Sun starts to shine reach Antarctica after six dark months. Just as the coldest part of night for us is just after dawn, so it is that Antarctica gets is coldest temperatures of the year around the equinox. It is the temperature difference across the Southern Ocean that energies the westerly winds found there, and so these at their most energetic, and expand to their furthest north, around the equinox. This is the origin of the term “equinoctial gales”, however I prefer the phrase ”gales of the Antarctic dawn”.

From Bob McDavitt at


Leave a Reply

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

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

Google photo

You are commenting using your Google 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 )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.