Where do you find the semidiameter...

🔍 Key Concepts

• Semidiameter correction applies to the apparent size of a body’s disk (like the Sun or Moon) when measuring from its limb, not its center • For stars and planets, we normally assume they are point sources with no visible disk in sextant work • Check what types of altitude corrections are listed in the Nautical Almanac and which bodies they apply to

💭 Think About

• Ask yourself: when do you actually see a measurable disk through the sextant – for the Sun, Moon, planets, or stars? • Look at the standard sequence of sextant altitude corrections: index error, dip, refraction, parallax, semidiameter – are all of those applied to stars? • Think about whether any fixed numerical semidiameter correction would make sense for stars, which appear as points rather than disks

✅ Before You Answer

• Verify which bodies have a tabulated semidiameter in the Nautical Almanac (Sun? Moon? planets? stars?) • Check whether the altitude correction tables in the Nautical Almanac list a separate semidiameter correction for stars • Confirm whether stars are treated as having an apparent disk size in standard sight-reduction practice

Where do you find the semidiameter correction to be used to correct sextant observations of the stars?

It is included in the altitude corrections inside the front cover of the Nautical Almanac.

Table 25 in Bowditch contains the correction.

A correction of -0.5' should be applied to all star sights.

No semidiameter correction is used.

🔍 Key Concepts

💭 Think About

✅ Before You Answer