Transcript Sextant Lessons presented by Gary Fix Amateur Astronomer

Sextant Lessons
presented by
Gary Fix
Amateur Astronomer
Visit www.garytheastronomer.com
Using the
Noon Site Method
Your location on earth is based on:
•
LATITUDE
•
•
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.
. Polaris
If you are near Miami:
• Altitude of Polaris is 26
• Latitude is 26 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
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.
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 60 degrees.
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
The Three Star Method
This involves measuring the Altitude of three
celestial objects (stars, planets, sun or moon in
three different directions at about the same time
to calculate your Longitude and Latitude. You
start with your Dead Reckoning location and
calculate the Altitude of the three objects. Then
compare the computed altitude to the Observed
Altitude and using the Azimuth (Direction) to each
object, you plot the intersection of the Intercepts
to get your FIX (Latitude and Longitude)
This Method takes about a year
of classroom instruction to
master the concept and
mathematical formulas
involved.
We will stick with the
NOON SITE Method
At sea, a typical Noon Site might look like this:
TIME
ALTITUDE
16:50
16:53
16:55
16:57
16:59
17:01
17:03
17:05
17:07
76d 34m
76d 54m
77d 05m
77d 08m
77d 10m
77d 10m
77d 08m
77d 05m
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 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
(the location 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).
You will notice a large section
of the Daily Pages in the
Nautical Almanac dedicated
to calculating Twilight along
with Sunrise and Sunset.
This is critical when sighting
stars and Planets.
Remember at sea you are
using the Horizon as your
reference plane so it must be
dark enough to see the stars
and light enough to see the
Horizon. This is TWILIGHT
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
Now, let’s go…..
IMPORTANT
Please use Hand Sanitizer
BEFORE and AFTER
handling the Sextant