Intercept is the difference between the observed height of celestial bodies and the calculated height calculated according to the assumed geographical position. Used to determine latitude and longitude. Mathematically, it refers to the (horizontal or vertical) coordinates of all intersections between a function and a coordinate axis, which can be any number.

The vertical position of sextant at sea is found by swinging sextant. When the sextant is swung, the celestial image can draw an arc with respect to the water antenna in the field of view of the telescope. When the celestial image is at the lowest point of the arc and located in the center of the telescope's field of view, it can be judged that the scale arc surface of the sextant coincides with the azimuth circle surface of the celestial image, that is, the sextant is in a vertical position.

If the predetermined tangent point of the celestial image is just tangent to the water antenna, the sextant reading will be the observed height of the celestial body after correcting the instrument error.

If the predetermined tangent point of the astronomical phenomena is pressed in the water at this time, the observation height will be greater than the astronomical phenomena height; When the predetermined tangent point of the astronomical phenomena is suspended in the air, the observation height will be smaller than the astronomical phenomena height.

After research, there are three ways to arc the celestial image by actually swinging the sextant:

1. The sextant swings around the optical axis OL of the telescope, and at the same time makes the optical axis of the telescope slightly deviate from the left and right of the azimuth circle of the celestial body, so as to keep the image of the celestial body in the center of the field of view of the telescope.

2. The sextant swings around the vertical line, that is, the scale cambered surface of the sextant is kept vertical, so that the optical axis of the telescope swings left and right along the waterline.

3. The sextant swings around the light from the celestial body to the moving mirror.

Using the assumed latitude and longitude method to observe celestial positioning, we can not only observe the orientation of celestial bodies, but also observe the height of celestial bodies. No matter day or night, we can measure celestial positioning at any time as long as we can observe celestial bodies.

At night, satellite positioning is no longer limited by the time of dawn, and multi-satellite azimuth positioning can be observed at any time; By observing the direction and height of the sun during the day, the latitude and longitude of the surveyor's position can be directly calculated, and it is no longer necessary to wait for the opportunity to move the line to locate.