static AstroVector DirectionVector(AstroTime time, Observer observer, double altitude, double azimuth)
{
// Convert horizontal angles to a horizontal unit vector.
var hor = new Spherical(altitude, azimuth, 1.0);
AstroVector hvec = Astronomy.VectorFromHorizon(hor, time, Refraction.None);
// Find the rotation matrix that converts horizontal vectors to equatorial vectors.
RotationMatrix rot = Astronomy.Rotation_HOR_EQD(time, observer);
// Rotate the horizontal (HOR) vector to an equator-of-date (EQD) vector.
AstroVector evec = Astronomy.RotateVector(rot, hvec);
return(evec);
}
static int HorizontalCoords(
out Spherical hor,
double ecliptic_longitude,
AstroTime time,
RotationMatrix rot_ecl_hor)
{
var eclip = new Spherical(
0.0, /* being "on the ecliptic plane" means ecliptic latitude is zero. */
ecliptic_longitude,
1.0 /* any positive distance value will work fine. */
);
/* Convert ecliptic angular coordinates to ecliptic vector. */
AstroVector ecl_vec = Astronomy.VectorFromSphere(eclip, time);
/* Use the rotation matrix to convert ecliptic vector to horizontal vector. */
AstroVector hor_vec = Astronomy.RotateVector(rot_ecl_hor, ecl_vec);
/* Find horizontal angular coordinates, correcting for atmospheric refraction. */
hor = Astronomy.HorizonFromVector(hor_vec, Refraction.Normal);
return(0); /* success */
}
static int CameraImage(Observer observer, AstroTime time)
{
const double tolerance = 1.0e-15;
// Calculate the topocentric equatorial coordinates of date for the Moon.
// Assume aberration does not matter because the Moon is so close and has such a small relative velocity.
Equatorial moon_equ = Astronomy.Equator(Body.Moon, time, observer, EquatorEpoch.OfDate, Aberration.None);
// Also calculate the Sun's topocentric position in the same coordinate system.
Equatorial sun_equ = Astronomy.Equator(Body.Sun, time, observer, EquatorEpoch.OfDate, Aberration.None);
// Get the Moon's horizontal coordinates, so we know how much to pivot azimuth and altitude.
Topocentric moon_hor = Astronomy.Horizon(time, observer, moon_equ.ra, moon_equ.dec, Refraction.None);
Console.WriteLine($"Moon horizontal position: azimuth = {moon_hor.azimuth:F3}, altitude = {moon_hor.altitude:F3}");
// Get the rotation matrix that converts equatorial to horizontal coordintes for this place and time.
RotationMatrix rot = Astronomy.Rotation_EQD_HOR(time, observer);
// Modify the rotation matrix in two steps:
// First, rotate the orientation so we are facing the Moon's azimuth.
// We do this by pivoting around the zenith axis.
// Horizontal axes are: 0 = north, 1 = west, 2 = zenith.
// Tricky: because the pivot angle increases counterclockwise, and azimuth
// increases clockwise, we undo the azimuth by adding the positive value.
rot = Astronomy.Pivot(rot, 2, moon_hor.azimuth);
// Second, pivot around the leftward axis to bring the Moon to the camera's altitude level.
// From the point of view of the leftward axis, looking toward the camera,
// adding the angle is the correct sense for subtracting the altitude.
rot = Astronomy.Pivot(rot, 1, moon_hor.altitude);
// As a sanity check, apply this rotation to the Moon's equatorial (EQD) coordinates and verify x=0, y=0.
AstroVector vec = Astronomy.RotateVector(rot, moon_equ.vec);
// Convert to unit vector.
double radius = vec.Length();
vec.x /= radius;
vec.y /= radius;
vec.z /= radius;
Console.WriteLine($"Moon check: x = {vec.x}, y = {vec.y}, z = {vec.z}");
if (!double.IsFinite(vec.x) || Math.Abs(vec.x - 1.0) > tolerance)
{
Console.WriteLine("Excessive error in moon check (x).");
return(1);
}
if (!double.IsFinite(vec.y) || Math.Abs(vec.y) > tolerance)
{
Console.WriteLine("Excessive error in moon check (y).");
return(1);
}
if (!double.IsFinite(vec.z) || Math.Abs(vec.z) > tolerance)
{
Console.WriteLine("Excessive error in moon check (z).");
return(1);
}
// Apply the same rotation to the Sun's equatorial vector.
// The x- and y-coordinates now tell us which side appears sunlit in the camera!
vec = Astronomy.RotateVector(rot, sun_equ.vec);
// Don't bother normalizing the Sun vector, because in AU it will be close to unit anyway.
Console.WriteLine($"Sun vector: x = {vec.x:F6}, y = {vec.y:F6}, z = {vec.z:F6}");
// Calculate the tilt angle of the sunlit side, as seen by the camera.
// The x-axis is now pointing directly at the object, z is up in the camera image, y is to the left.
double tilt = RAD2DEG * Math.Atan2(vec.z, vec.y);
Console.WriteLine($"Tilt angle of sunlit side of the Moon = {tilt:F3} degrees counterclockwise from up.");
IllumInfo illum = Astronomy.Illumination(Body.Moon, time);
Console.WriteLine($"Moon magnitude = {illum.mag:F2}, phase angle = {illum.phase_angle:F2} degrees.");
double angle = Astronomy.AngleFromSun(Body.Moon, time);
Console.WriteLine($"Angle between Moon and Sun as seen from Earth = {angle:F2} degrees.");
return(0);
}
