Sextant Lessons presented by Gary Fix Amateur Astronomer Using the Noon Site Method

LATITUDE LONGITUDE Your location on earth is based on: Varies from 0 at the equator, to 90 at the North and South Poles LONGITUDE Varies from 0 at Greenwich, to 180 East and West

Navigating at Sea: Sailors have always needed to determine their location during the day, and at night. Sailors would determine LATITUDE by measuring the altitude of Polaris (the North Star).

Polaris (the North Star): Directly over the North Pole Appears to be stationary Sun, moon, planets, and stars rise in the East and set in the West around it.

Latitude is 26 North of the Equator. If you are near Miami: Altitude of Polaris is 26 Latitude is 26 North of the Equator. If you are near Boston: Altitude of Polaris is 42 Latitude is 42 North of the Equator. If you are at the North Pole: Polaris is directly overhead Altitude is 90 Latitude is 90 North of the equator. You can find Polaris by using the last two stars in the cup of the BIG DIPPER and extending their direction and distance four times to the last star in the handle of the LITTLE DIPPER

Finding LONGITUDE is difficult, because it is a function of TIME. Finding a ship’s location was very difficult. Good, sea-faring chronometers (clocks) were not available. Royal Navy offered 20,000 English Pounds as a prize for creating an accurate timepiece for sailors at sea. In 1740, John Harrison invented a clock that was accurate to 3 seconds a month. Interestingly, he died before collecting any money.

Steps to finding LONGITUDE: Use LOCAL NOON at Greenwich, England Find when LOCAL NOON occurred where you are The difference between the two is your longitude. Notes: Local Noon is when the sun is at its highest point in the sky. Remember it takes the Sun exactly 24 hours to make a complete rotation around the Earth.

The Sextant is an instrument for measuring the angle between the Horizon and any Celestial Object (Sun, Moon, Planets, Stars).

Because of the Earth’s Rotation, all Celestial Objects (Sun, Moon, Planets and Stars): Appear to rise in the East Set in the West Reach their highest Point (TRANSIT), at due South The NOON Sextant Site is measured when the SUN is: At it’s highest Altitude (Local Noon) Is due South of the Observer

What do Lewis and Clark, and Roald Amundsen have in common with MacGyver? Obviously, they didn’t all have duct tape, or bad mullet hairstyles. Let’s set the scene: It’s 1804. Lewis and Clark need to explore their way to the Pacific Ocean. It’s 1911. Amundsen needs to explore the South Pole. They need to know the sun’s altitude. They can’t see the horizon. They can’t carry heavy equipment.

Lewis and Clark, Roald Amundsen, and MacGyver all used common, everyday objects to solve a problem. At sea, you use the ocean’s horizon as a reference line. On land, you must use an Artificial Horizon: Lewis and Clark used a sheet of glass floating on water. Amundsen used a pool of Mercury in a shallow pan. Today, Mercury is too dangerous. We’ll use the water method.

How to use an ARTIFICIAL HORIZON: Place the tray on a flat, motionless surface. Add water, then add glass. Use sextant to measure angle to the sun. Divide observed Altitude by 2 to get real Altitude. Notes: The Arc on the Sextant only reads to 120 degrees. You can only measure the Sun until it is 60 degrees high. (120 divided by 2) This is fine during winter. During summer, altitudes are often greater than 120. Actual Horizon

There are several ways to use a sextant, some are easy and some are complicated The easiest method is the NOON SITE

The Lunar Distance Method For several days each month the Sun and the Moon are both visible at the same time You must measure the altitude of the sun and the altitude of the Moon and the angle between them. You can then calculate your Latitude and Longitude. However, without a computer this takes about 6 pages of hand calculations to compute

With your right hand, grab the handle on the Sextant

Put your left hand on the Micrometer wheel. By turning this knob, the image of the Sun /Stars will appear to move up and down in the half-glass /half-mirror window.

Make sure the colored filters are in place when looking at the Sun: Use one or two on a cloudy day, Use all four on a bright sunny day, Push them aside when looking at stars or planets

Read Degrees on the circular arc. Read Minutes on the Micrometer Drum

At sea, a typical Noon Site might look like this: TIME ALTITUDE 16:50 76d 34m 16:53 76d 54m 16:55 77d 05m 16:57 77d 08m 16:59 77d 10m 17:01 77d 10m 17:03 77d 08m 17:05 77d 05m 17:07 76d 54m 17:00 will be Local Noon NOTE: Time is in 24-hour format and set to Greenwich Time. Remember that altitude (77d 10m) is not used to calculate longitude. Altitude is only used to calculate latitude. Remember this number for later! Let’s use the time to find our longitude!

Let’s take that altitude to find our latitude! Let’s estimate 17:00 as Local noon. It’s altitude would be 77d 10m. 17:00 - 12:00 = 5 hours 5 hours X 15 degrees per hour = 75 deg (15 degrees per hour x 24 hours = 360 degrees / 1 day) So, you would be at 75 deg west longitude from Greenwich. Let’s take that altitude to find our latitude!

To get your latitude, you will need an Almanac that lists the locations of the sun, planets, and stars for each hour of the day, 365 days a year. You will notice I am using a Nautical Almanac from 1985. The data is nearly identical to 2014. This version is free on the Internet.

Here is an example page from my Almanac: Let’s look at June 24th. Let’s look at this day more closely!

GMT: Greenwich Mean Time GHA: Greenwich Hour Angle (the location of the Sun Relative to Greenwich) Dec: Declination of the Sun above or below the Equator) The declination for the 24th of June at 17:00 hours is 23 degrees and 24.4 minutes north of the Equator.

If we take the altitude, and subtract it from 90 degrees: 90 – 77d 10min = 12 deg 50 min Add the declination from the table: 12 deg 50 min + 23 deg 24.4 min = 36 deg 14.4 min north latitude of the equator

To get a truly accurate location you must make a series of corrections to your Observed Altitude: Instrument error mirrors not perpendicular to arc Refraction light rays bend downward thru the atmosphere Parallax all table books are based on locations at the center of the Earth Semi-Diameter if you are using the edge of the Sun / Moon instead of the center Dip at sea if you are high above the water, the Horizon is below True Level

What does the sun and an ice skater have in common? No, it has nothing to do with gas. They both make figure 8’s! This shows the location of the Sun at NOON for a whole year. The top is the first day of summer (Jun 21st). The bottom is the first day of winter (Dec 21st).

Now, let’s go…..

Please use Hand Sanitizer BEFORE and AFTER handling the Sextant IMPORTANT Please use Hand Sanitizer BEFORE and AFTER handling the Sextant