MGN 69 Topic 1 (h) – to use a sextant, identify and correct errors;
For Sextant, you should know How to use a sextant, Identify its parts, limitations, errors and how to correct them.
Why we use Sextant? What is Sextant?
A sextant is a precision instrument which is used to measure the altitude of a celestial body and VSA (Vertical sextant angle) and HSA (horizontal sextant angle) of a terrestrial body. [angular distance between two visible objects. Eg; astronomical object and horizon or two fix points on land]
“What are Azimuth and Amplitude? Can we use Sextant for this two?”
Principle:
When a ray of light is reflected from two mirrors in succession, then the angle between the first and last directions of the ray is twice the angle between the mirrors.
Correctable errors: [PSI – Perpendicularity-Side-Index]
→ Perpendicularity: When Index mirror is not ⊥ perpendicular to the plane of the instrument
→ Side error: When Horizon mirror is not ⊥ perpendicular to the plane of the instrument.
→ Index Error: When the Index and Horizon mirror is not (∥) parallel to each other.
How to Correct Error of Perpendicularity?
1) Bring index bar on the middle of the arc.
2) Hold sextant horizontally, arc away from you, and look at index mirror.
3) The arc must appear as one single line.
4) Adjust by a screw on the back of the index mirror.
How to Correct Side Error?
Index bar at zero.
Non-Adjustable / Non-correctable Errors: We can’t correct these errors.
“All non-correctable errors will be mentioned in the certificate inside the sextant box”
- Shade error: Faces of the shades not parallel to the ground.
- Prismatic error: When two faces of the mirror not parallel to each other.
- Graduation error: Incorrect scale on the arc, Vernier or micrometre.
- Worm & Rack: damage to the gearing rack.
- Centring: Pivot of index arm is not at the centre of the curvature of the arc.
- Collimation: Axis of telescope not parallel to plane of the instrument.
Principle Explanation:
- “For a sextant, the light is reflected twice in the same plane from the index mirror to the horizon mirror then again from the horizon mirror to the telescope. In these circumstances, the angle between the first and last direction of the ray of light is twice the angle between the index and horizon mirrors.”
- The principle is based on the first law of light–“when a ray of light is reflected from a plane mirror, the angle of incidence of the ray equals the angle of reflection.
Example Orals Question:
- State the principle of a sextant.
- Name the first/second/third adjustable error.
- Correct the first/second/third adjustable error.
- Correct the first adjustable error using horizon/star. Explain why re-adjustment of second error may be necessary.
- Obtain index error using the sun method.
- Explain parallax when: –
– Using a roof line to demonstrate index error.
– Using two very close objects for HAS - Explain how to improve horizon.
- Name some non-adjustable errors.
- State where the non-adjustable errors are collated.
- Read the micrometre “on” and “off” the arc.
- Take a VSA and HAS.
- State the purpose of the telescope eyepiece.