MARITIME – CELETIAL NAVIGATION CALCULATOR
Sha Star
A precise tool.
What is the Sha Star & How does it work?
In celestial navigation the Sidereal Hour Angle (SHA) is the angular distance measured westward from the Greenwich meridian to the hour circle of a celestial body. It is analogous to longitude on the Earth’s surface, but it is expressed in degrees, minutes, and seconds of arc. The SHA of a navigational star is directly taken from the Greenwich Hour Angle (GHA) listed in the Nautical Almanac for the exact time of observation. Because the Earth rotates 15° per hour, the GHA (and therefore the SHA) changes continuously, making accurate time‑keeping essential. Once the SHA is known, it can be combined with the observer’s assumed longitude to compute the Local Hour Angle (LHA), which is the key quantity used in the sight reduction process to determine a line of position.
text{SHA};=;text{GHA}
SHA = Sidereal Hour Angle; GHA = Greenwich Hour Angle
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Result
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Frequently Asked Questions
What is the difference between GHA and SHA in celestial navigation?
The GHA is the angular distance of a celestial body measured westward from the Greenwich meridian at a specific time. The SHA is similar but directly taken from the Nautical Almanac for the exact observation time.
How do I use this calculator to find the SHA?
Enter the GHA of the celestial body and the exact observation time, then click calculate to get the corresponding SHA.
Why is the Earth’s rotation rate important for calculating SHA?
The Earth rotates 15° per hour, which affects the calculation of both GHA and SHA, ensuring accurate celestial navigation.
Can I use this calculator for any celestial body?
Yes, as long as you have the GHA from the Nautical Almanac for the specific celestial body and observation time.
What is the purpose of knowing the SHA in navigation?
Knowing the SHA helps navigators determine the position of a ship relative to celestial bodies, aiding in accurate positioning at sea.
How often should I update the GHA values for calculating SHA?
GHA values change with time, so it’s important to use the most current values from the Nautical Almanac for your observation.
Is there a specific formula to calculate SHA from GHA?
Yes, the SHA is directly taken from the GHA listed in the Nautical Almanac for the exact time of observation without additional calculations.
Results are for informational purposes only and do not constitute professional advice.