MARITIME – CELETIAL NAVIGATION CALCULATOR
Gha Sun
A precise tool.
What is the Gha Sun & How does it work?
The Greenwich Hour Angle (GHA) of the Sun is the angular distance measured westward from the Greenwich meridian to the hour circle passing through the Sun. It is a fundamental quantity in celestial navigation because it links the observerβs local hour angle to the universal reference frame of Greenwich. To obtain the GHA at a specific moment, navigators first determine the Sunβs right ascension (Ξ±) from an almanac and then compute the local sidereal time at Greenwich (LST). The relationship is expressed as GHA = LST β Ξ±, reduced to the range 0Β°β360Β°. Because LST advances approximately 360.9856Β° per mean solar day, the GHA changes continuously. Accurate calculation requires converting the UTC date and time to the number of days since the epoch J2000.0 and applying the sidereal rate.
GHA = LST – \alpha
GHA = Greenwich Hour Angle, LST = Local Sidereal Time at Greenwich, \alpha = Right Ascension of the Sun
Parameters
Result
β
Frequently Asked Questions
What is the purpose of calculating the GHA of the Sun?
The GHA of the Sun is used in celestial navigation to determine the observer's position relative to the Sun.
How do I obtain the right ascension (Ξ±) needed for GHA calculation?
You can find the right ascension from a nautical almanac or an astronomical database.
What is local sidereal time, and why is it important in this calculation?
Local sidereal time (LST) at Greenwich is the hour angle of the vernal equinox, used as a reference point for celestial coordinates.
Can I use this calculator for any celestial body besides the Sun?
This specific calculator is designed for the Sun. For other celestial bodies, different calculations are required.
What is the relationship between GHA and local hour angle in navigation?
The local hour angle of a celestial body is calculated by subtracting the observer's longitude from the GHA of that body.
How accurate does this calculator need to be for maritime navigation?
For maritime navigation, accuracy should be within 1 degree to ensure safe and effective positioning.
Is there a specific time frame during which I should calculate the GHA of the Sun?
GHA calculations are typically done at regular intervals throughout the day to track the Sun's position accurately.
Results are for informational purposes only and do not constitute professional advice.