An “Octant” THE PURPOSE OF SEXTANT ALTITUDES The entire purpose of taking a sextant altitude is to get a “position line”, and that is all we get A position line runs at right angles (i.e @ 900) to the bearing of a celestial body at the time of taking a sextant altitude The time at which we take this sextant altitude will dictate the type of position line we obtain THE PURPOSE OF SEXTANT ALTITUDES A sextant altitude at Meridian Passage will give us our latitude, but not a corresponding longitude A sextant altitude taken at any other time will give a position line only Nonetheless, when we take a sextant altitude there are a number of corrections which we must apply in order to obtain the True Altitude of the body Corrections are necessary as the observed altitude is taken from the surface of the earth instead of at the centre and… In the case of the sun, the altitude of the edge i.e upper or lower limb of the sun is taken rather than it’s centre Zenith H of E Centre of Earth Visible Horizon Zenith H of E Visible Horizon Centre of Earth Visible Horizon Zenith H of E Visible Horizon Centre of Earth Sensible Horizon Zenith 900 Sensible Horizon H of E Visible Horizon Centre of Earth Zenith Sensible Horizon Visible Horizon H of E True Altitude Rational Horizon Centre of Earth Index Error:Error:Is an instrument error causing the sextant to read too high or too low It may be given as + or - , or as “on the arc” or “off the arc” Dip:Dip:The observer is usually well above the surface of the earth and is therefore looking downwards to the visible horizon A negative correction is applied to bring the angle up to the plane of the sensible horizon This correction is obtained from the very front pages of the Nautical Almanac as follows Altitude Correction Table A2 at the front of the Nautical Almanac • Corrections are given for HoE in metres & feet • There are three columns i.e HoE in metres; HoE in feet; the dip correction • The table is entered with the HoE (feet or metres) • Please note the range of the tables; also interpolation will be required in certain cases • If the HoE matches a value in the tables then the preceeding correction is to be taken Diagram Zenith Sensible Horizon H of E Visible Horizon Rational Horizon Centre of Earth DIP Terms & Definitions Refraction:Rays of light from the body to the observer are “bent downwards” as they pass thru’ the earth’s atmosphere This makes the altitude seem greater than it actually is and so a negative correction is applied Altitudes of 100 or less should be avoided due to abnormal refraction Terms & Definitions Semi-Diameter:Since the sun appears as a disc rather than a point of light the observer must measure to the edge, or limb of the sun for an accurate measurement The altitude angle is taken as ‘upper limb’ or ‘lower limb’ (the top or bottom of the sun’s disc) and the semi-diameter applied to obtain the correct angle Terms & Definitions Semi-Diameter (continued):The values for semi-diameter are given in the daily pages of the Nautical Almanac and must be subtracted from Upper Limb observations and added to Lower Limb observations HOWEVER! Corrections for refraction and semi-diameter are combined into one correction which we call the Total Correction which is obtained from the Nautical Almanac • Total Correction is given for the Sun’s Upper & Lower limbs for the periods October ~ March and April ~ September • Also for the Stars and Planets • Please also take note of the additional correction for the planets Venus and Mars Enter the tables with App.Alt Terms & Definitions Stars and planets, being so far away, appear as points of light and not have a correction for semi-diameter Mars and Venus however, may have an additional correction due to their varying proximity to the earth and their altitude These will be found in the Altitude Correction Tables at the front of the N.A Terms & Definitions Semi-Diameter (continued):The value for the sun changes as the distance of the earth from the parent body changes during it’s yearly orbit around it Earth at Perihelion (closest) January 2nd SD = 00 16.3’ Earth at Aphelion (farthest) July 4th SD = 00 15.8’ Procedure for Altitude Correction (Sun Lower Limb; November) • • • • • • • Sextant Altitude(Sun) Index Error (I.E.) Observed Altitude Dip(HofE 24m) Apparent Altitude Total Correction True Altitude : : : : : : : 450 25.5’ 00.2’ 450 25.7’ was the IE on or off the arc? 8.6’ 450 17.1’ 000 15.3’ 450 32.4’ [...]... January 2nd SD = 00 16.3’ Earth at Aphelion (farthest) July 4th SD = 00 15.8’ Procedure for Altitude Correction (Sun Lower Limb; November) • • • • • • • Sextant Altitude( Sun) Index Error (I.E .) Observed Altitude Dip(HofE 24m) Apparent Altitude Total Correction True Altitude : : : : : : : 450 25.5’ 00.2’ 450 25.7’ was the IE on or off the arc? 8.6’ 450 17.1’ 000 15.3’ 450 32.4’ ... earth and their altitude These will be found in the Altitude Correction Tables at the front of the N.A Terms & Definitions Semi-Diameter (continued):The value for the sun changes as the distance of the earth from the parent body changes during it’s yearly orbit around it Earth at Perihelion (closest) January 2nd SD = 00 16.3’ Earth at Aphelion (farthest) July 4th SD = 00 15.8’ Procedure for Altitude Correction... Centre of Earth Zenith Sextant Altitude H of E Visible Horizon Centre of Earth Observed Altitude: Is the angle measured between the body and the visible horizon after the Sextant Index Error has been applied Apparent Altitude: Is the angle between the limb of the body and the sensible horizon Zenith Apparent Altitude Sensible Horizon Visible Horizon H of E Centre of Earth True Altitude: Is the angle between... limb’ or ‘lower limb’ (the top or bottom of the sun’s disc) and the semi-diameter applied to obtain the correct angle Terms & Definitions Semi-Diameter (continued):The values for semi-diameter are given in the daily pages of the Nautical Almanac and must be subtracted from Upper Limb observations and added to Lower Limb observations HOWEVER! Corrections for refraction and semi-diameter are combined... correction is obtained from the very front pages of the Nautical Almanac as follows Altitude Correction Table A2 at the front of the Nautical Almanac • Corrections are given for HoE in metres & feet • There are three columns i.e HoE in metres; HoE in feet; the dip correction • The table is entered with the HoE (feet or metres) • Please note the range of the tables; also interpolation will be required in... earth’s atmosphere This makes the altitude seem greater than it actually is and so a negative correction is applied Altitudes of 100 or less should be avoided due to abnormal refraction Terms & Definitions Semi-Diameter:Since the sun appears as a disc rather than a point of light the observer must measure to the edge, or limb of the sun for an accurate measurement The altitude angle is taken as ‘upper... Visible Horizon H of E Centre of Earth True Altitude: Is the angle between the centre of the body and the rational horizon at the centre of the earth Zenith Sensible Horizon Visible Horizon H of E True Altitude Rational Horizon Centre of Earth Index Error:Error:Is an instrument error causing the sextant to read too high or too low It may be given as + or - , or as “on the arc” or “off the arc” Dip:Dip:The ... (closest) January 2nd SD = 00 16.3’ Earth at Aphelion (farthest) July 4th SD = 00 15.8’ Procedure for Altitude Correction (Sun Lower Limb; November) • • • • • • • Sextant Altitude( Sun) Index... November) • • • • • • • Sextant Altitude( Sun) Index Error (I.E .) Observed Altitude Dip(HofE 24m) Apparent Altitude Total Correction True Altitude : : : : : : : 450 25.5’ 00.2’ 450 25.7’ was the IE... time of taking a sextant altitude The time at which we take this sextant altitude will dictate the type of position line we obtain THE PURPOSE OF SEXTANT ALTITUDES A sextant altitude at Meridian