The most well-known use for a sextant is to measure the altitude of celestial bodies so that you can plot your position onto a chart.
In order to get the most accurate sight, however, you need to make sure you are ready to take your sights at the optimal time.
The best time to use a sextant for celestial navigation is 30 minutes before sunrise or 30 minutes after sunset. It should be dark enough to observe navigational stars, yet light enough to have a clear horizon visible.
You just need to remember that a sextant is just a scientific instrument, used to measure the angle between two different objects.
We won’t go into detail about how the sextant works in this article, but if you are interested you can check out my other article: How Sextants Work: An Illustrated Guide
For a traditional star fix, a sextant just needs to measure the angle between the star and the horizon.
To get the most accurate measurement, both the star and the horizon need to be clearly visible.
It is a fine balance because the horizon is clearly visible during the day, but no stars are visible. Conversely, stars are clearly visible at night, yet the horizon is not.
As a compromise, navigators use the period of time between dawn and sunrise to take morning sights, and between sunset and dusk to take evening sights.
It is known as twilight and lasts for approximately 40 minutes.
The best time to take your sights is approximately 10 minutes after the sky starts to get light in the morning, or 10 minutes before the sky gets completely dark in the evening. This is approximately 30 minutes before sunrise and 30 minutes after sunset.
It is still possible, however, to use a sextant at other times as well.
What times can you use a sextant?
The best time to use a sextant varies depending on the type of fix you are trying to obtain.
Pro Tip: To find out how to use the sextant to obtain these different fixes, check out my complete guide: How To Use A Sextant: A Step By Step Guide
Best time to take morning sights using a sextant
The best time to take morning sights with a sextant is 30 minutes before sunrise, during Nautical Twilight.
Navigational stars will still be visible in the sky, but the sun will be starting to illuminate the horizon.
Start by taking sights in the East, and finish by taking sights in the West as the sun rises.
Best time to take evening sights using a sextant
The best time to take evening sights using a sextant is 30 minutes after sunset, during Nautical Twilight.
You’ll want to be set up to take your sights while it is still light enough to see and be ready to take your sights as soon as stars become visible.
At 30 minutes after sunset, you should be able to see navigational stars in the sky, yet the horizon will still be visible.
Stars in the East will be the first ones that become visible, so shoot those ones first. The ones in the West will appear latest, so finish your evening sights with those.
Best time to take a noon sight using a sextant
The only time to take a noon sight is at the celestial noon of the body you are using.
For the sun, this will be the point that the sun is at the highest point in the sky.
You need to calculate the time of noon using your nautical almanac and be ready to take the sight at that time.
It doesn’t usually happen at 12 o’clock, but that is a good starting point if you have nothing else to work from.
Best time to take a sun sight using a sextant
The best time to take a sun sight using a sextant is when the sun is between 30° and 60° high in the sky.
You can take a sun-sight at any time during the day, but the highest accuracy is achieved when the sun is between 30° and 60°.
Best time to take a vertical sextant angle
The best time to use a sextant to obtain a vertical sextant angle is during the day when there is enough light to see both ends of the object you are measuring.
For example, if you are taking a vertical sextant angle of a lighthouse, you need enough light to be able to see the sea at the bottom of the lighthouse so that you can measure the angle between sea level and the focal plane of the light.
Best time to take a horizontal sextant angle
Horizontal sextant angles can usually be taken any time during the day or night.
During the day, you can measure the horizontal angle between two visible objects.
At night, you can measure the horizontal sextant angle between two lights.
The most important thing is to be able to positively identify the objects that you are measuring so that you can plot them accurately on your chart.
Can you use a sextant during the day?
While the most well-known use of the sextant is to take sights of the stars in a dark sky, it is still perfectly reasonable to use it during the day. You just need to be able to see the objects you are measuring.
During the day, you can use a sextant to take celestial sights of the sun, the moon or bright planets. You can also take terrestrial observations of buildings, cathedrals, bridges and distinctive land features using vertical or horizontal sextant angles.
It is less common to use a sextant during the day, but a thorough understanding of your instrument will open up countless possibilities for ways it can be used.
Pro Tip: Learn how to make the best use of your sextant in this article: 7 Tips To Use A Sextant Like A Pro
Once you understand how the sextant works and you practise using it, you will find that there are many opportunities for getting it out and having a practise, regardless of the time of day or night.