Observing the Sun by looking directly into it can be extremely dangerous. Since excessive exposures can cause cataracts from the UV radiations and burnt retinas from the visible light, caution must be taken while observing the Sun, to protect the viewer's eyes.
A dependable Sun filter should be added to the Sextant to accommodate this preventative health measure, otherwise alternative reference objects should be viewed.
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1. Observe the Sun for only a few seconds and only after the shade is in place.
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2. Never stare directly at the Sun.
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3. If any discomfort is sensed, discontinue observing the Sun until a stronger filter.
Materials that can be used to make the filter:
-
- Aluminized Mylar film - this is a material specifically developed for solar observation. Can be found in science supply stores. Probably the best material available.
-
- Welder's glass - strong filter, used to protect the welder eye. Difficult to cut, can be found in construction stores.
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- Photography film. Use a dark negative with silver coating. This means black-and-white film. Color film does not contain silver and will not filter the UV rays (they are dark for visible light, but not for UV light).
-
- Dark floppy disk media.
Additional eye safety information is available at:
http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/safety2.html
Photographic 35 mm dark negative film (there is one in the end of every film roll) is one possibility as shades for Sun and Moon sights. The negatives should be mounted in slide frames: two layers of photographic 35 mm dark negative film for the Sun frame and one layer of photographic 35 mm dark negative film for the Moon. Both slide frames are removable and are attached to the instrument frame using Lego pieces. Trimming the lower edge of the slide to make it thinner will ensure that the slide window matches the imaginary "tube" formed by the mirror edges.
The shade must be positioned between the two mirrors and the filter surface must be orthogonal to the line connecting both mirror centers. This is to prevent introducing a refraction error. Try to position the slide center in the line connecting the two mirror centers. The Sun observation is made by looking thru the half silvered mirror, below the shade.
(image available in print form)
Figure 8
, Source: [8]
Sun filters.
Fine-tuning the mirror angles:
This Sextant is not equipped with screws to adjust the tilt of the mirrors. Instead, each mirror can be fine-tuned by inserting paper shims between the Lego brick and plates (or by reducing the Lego brick and plates by sanding) at meaningful locations to achieve the desired results.
First check the angle of the CD mirror (center mirror). As you look to this mirror, the reflected CD edge must be perfectly aligned with the edge you see outside the mirror. This alignment should be performed for all directions.
The half-silvered mirror should be trimmed by setting the instrument to 0°00', focusing on a distant object and adjusting the mirrors so that the direct and reflected images align. Adhere a round 1x1 Lego piece to the CD surface, to use as a turning knob.
(image available in print form)
Figure 9
, Source: [8]
Using the CD Sextant by viewing the Sun and the horizon.
1. The
Algebra
Lessons.
-
a. The
Intercept Method
, or
Marc St Hilaire Method
-
Title: Calculating Locations by The Intercept Method, or Marc St Hilaire Method.
-
Time: The time allocated is approximately 90 minutes.
-
Materials: Graphic Calculator; CD Sextant; Commercial Plotting Sheets.
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Objectives: Students will be able to locate a position by sighting with the CD Sextant, calculating the algebra with the Graphic Calculator, and plot that position on the Commercial Plotting Sheets by The Intercept Method, or Marc St Hilaire Method.
The sextant is an instrument that is applied to measure these angles. The eyepiece is aligned to the small mirror, which is fixed in the frame of the instrument. This mirror is half transparent. Through the transparent half, the navigator can see the horizon directly. The small mirror also partially reflects the image from the large mirror, where the star is visible. The large mirror is mobile and turns with the arm of the sextant. Altering the angle between the two mirrors aligns the navigational sighting. The altitude of the star is measured in the Vernier scale. There is a drum to make the fine adjustments. Whole degrees are read in the scale and the minutes in the drum.
(image available in print form)
Figure 10
, Source: [8]
The Earth and the Celestial Sphere.
If the Earth were the center of the universe, then around the Earth there could be a larger sphere centered in the same point, upon which the stars are fixed as if they were painted across its internal surface. The
Celestial Sphere
(
figure 10
) is this other sphere
.
(image available in print form)
Figure 11
, Source: [8]
Earth coordinate system.
To specify a position on the surface of the Earth, a system of coordinates has been developed that consists of two angles: latitude and longitude.
Latitude
is the angle measured from the Equator in direction North-South.
Longitude
is the angle in the Pole between the Meridian of Greenwich and that of the considered position (
figure 11
).
(image available in print form)
Figure 12
, Source: [8]
Celestial Coordinate System
A similar system is used for the Celestial Sphere (
figure 12
). The angle analogous to the latitude in the celestial sphere we call
declination
. The declination is measured in the plane North-South, from the Celestial Equator. The analog to the longitude is named
Right Ascension
or
RA
. Like the longitude, the Right Ascension is measured from an arbitrary Meridian: the Vernal Equinox Point (a.k.a. first point of Aries).
To determine an astronomical position, draw the lines of position:
-
1. Plot your assumed position.
-
2. Using a parallel ruler, draw a line passing on the assumed position, in the direction of the Azimuth of the star.
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3. Over this line, measure the error Delta of the estimate - in the direction of the star or contrary to it - according to the sign of the Delta.
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4. Draw the line of position, orthogonal to the Azimuth, at this point.
For more information:
http://www.kwsi.com/ynhti/SextantWebsite/