🔍 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