Principles of Celestial Navigation

A Day in the Life of a Navigator » Deneb Sight Reduction

Here is the strip form for our three-star fix. For Capella, the sextant altitude (Hs) is noted and the sight reduction has been completed. The sextant altitudes and index and dip corrections for Deneb and Fomalhault are also shown on the form.

strip form for Capella, Deneb, and Formalhaut, for 0800 UT morning three-star fix

From the Nautical Almanac, the appropriate Altitude Correction for Deneb is -0.8 arcminutes.

Based on the values, select the correct Ho value for Deneb:

Observed altitude (Ho) for Deneb:

To find the observed altitude Ho, add the index correction and dip correction, and apply them to the sextant altitude, Hs. Then find the altitude correction and apply that value to obtain Ho. Subtracting degrees and minutes might appear somewhat complex, but it is not. In the case of Deneb, the index correction and dip correction sum to -3.5'. We apply that value to the sextant altitude, Hs. Remember that 1 degree = 60 minutes, so we can always make the subtraction easier by adding 60 minutes to the minutes value and taking 1 degree away from the degrees values.

For Deneb, the corrections applied to Hs can be shown as 49° 02.1' - 3.5' = 48° 62.1' - 3.5' = 48° 58.6'

Then, apply the altitude correction: 48° 58.6' - 0.8' = 48° 57.8'

Here is what the your results for Deneb look like on the partially completed strip form.

strip form for Capella, Deneb, and Formalhaut, for 0800 UT morning three-star fix

You can now use the USNO website at to find the computed altitude (Hc) for Deneb at 0800 UT 17 July 2018. Remember, your assumed position is the DR and equals 31° 25.0 minutes N, 054° 12.0 minutes W. Once you have located the computed altitude (Hc), enter that value here.

Enter the azimuth value (Zn) for Deneb here:

The computed altitude (Hc) for Deneb should be 48 degrees 52.8 minutes and azimuth is 304.6 degrees. If you did not get the correct values, or if you still have questions about observed altitude (Ho) versus computed altitude (Hc), you may wish to review the previous content and then work through the exercise again.

How do Ho and Hc compare for the star Deneb?