Predicting planets

The Nautical Almanac is full of so much information that it requires a Baedeker for the novice — and that includes those who have used the book many times. Most celestial navigators primarily take observations of the sun and become familiar with the daily sun pages and all the ephemera having to do with sun sights. For instance did you know that it is a simple matter to decipher the position of the four navigational planets, so that they can become available for observations?

Except for Pluto, which has recently lost its planet standing, there are eight planets including home base. Of these, celestial navigators rely on only four, Venus, Mars, Jupiter and Saturn, as signposts in the sky.

There are two methods in the Nautical Almanac of finding which planets are suitable for a sight. The first method is found on the bottom right hand of the daily planet pages. Venus, Mars, Jupiter and Saturn are listed along with their SHAs and the time in local time of their Meridian passage. Let’s look at an example. We will use June 6, 2014, and want to find which planet can be seen at either civil or nautical twilight. Remember when observing any celestial object other than the sun, you need to have enough darkness so the object is visible, combined with enough light, so that the horizon can be seen as well.

The GHA and declination of the four planets are listed on the daily pages on the planet page; the times are also listed for meridian passage in local time in the bottom right hand corner of the page. Planets are relatively easy to find, first because unlike stars they don’t “twinkle” and because they travel in an orbit that is similar to the apparent path of the sun.

On June 6, 2014, the meridian passage of Mars takes place at 19 hours 37 minutes local time. The declination of Mars is south 4°. This means it will be lower in the sky than the sun (whose declination on that day is north 22°), so you will be looking for a bright reddish object below where you have observed the path of the sun during the day.

The meridian passage of Saturn occurs at 2200 local time. By that time it is already dark so you would be looking more to the east at twilight and since the declination is south 14°, to the south of the apparent path of the sun. Because more than one planet is usually visible it is a relatively easy matter to obtain a fix instead of just an LOP. Planets are also easier to calculate than stars and the moon and yield excellent results once you get the hang of them.

The other method of predicting what planets are available is in the front pages of the Nautical Almanac. On page 9 there is a graph that shows the relationship of the planets by month to local apparent time and the rising and setting of the sun. In this graph Mercury is included, but in my experience, Mercury being so close to the sun, rises and sets at about the same time and the sky is already too bright to shoot it. I have never known anyone to take an observation of Mercury. You can see from the graph that Mars on June 6 has a meridian passage of about 1900 hours. And Saturn of about 2200 hours confirming what was found in the daily pages. Remember you don’t have to shoot a planet only at meridian passage, it can either be before or after, the reference to meridian passage just gives you an idea of where the planet is in the sky. Also check the planet’s declination so you can find it more easily. Good luck!

By Ocean Navigator