a reading on a sailboat shows that on December 21, the noonday Sun has an altitude of 86.5 degrees to the south. and the marine chronometer reads 7 a.m. wht are the latitude and longtitude of the ship.

To determine the latitude and longitude of the sailboat, we can use the altitude of the noonday sun and the time difference between the local noon and the marine chronometer's reading. Here's how you can calculate it step by step:

Step 1: Calculate the Local Apparent Noon (LAN)
The Local Apparent Noon is the time when the sun reaches its highest point in the sky. It is given by the equation:
LAN = 12:00 - (Chronometer Reading / 15)

In this case, the chronometer reading is 7 a.m.
Therefore, LAN = 12:00 - (7 / 15) = 11.53 a.m.

Step 2: Calculate the Solar Declination
The solar declination is the angle between the equatorial plane and a line perpendicular to the Earth's orbit around the sun. On December 21st, the solar declination is approximately -23.5 degrees.

Step 3: Calculate the Zenith Distance
The zenith distance is the angular distance between the observer's zenith (directly overhead) and the position of the sun. It can be calculated using the following formula:
Zenith Distance = 90 degrees - Altitude of the Sun

In this case, the Altitude of the Sun is given as 86.5 degrees to the south.
Zenith Distance = 90 - 86.5 = 3.5 degrees

Step 4: Calculate the Latitude
The latitude can be determined by the following equation:
Latitude = 90 degrees - Zenith Distance + Solar Declination

In this case,
Latitude = 90 - 3.5 + (-23.5) = 63 degrees South

Step 5: Calculate the Longitude
To determine the longitude, we will need more information. The given data doesn't provide enough details to directly calculate the longitude.