Orizzonte artificiale

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import javax.swing.JPanel;

public class JHorizon extends JPanel {

    private int h = 0, a = 0;

    public JHorizon() {
        setMinimumSize(new Dimension(100, 100));
        setPreferredSize(new Dimension(100, 100));
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);

        double m = Math.tan(Math.toRadians(a));
        int y1 = (int) (h - m * 50), y2 = (int) (h + m * 50);
        int[] x = new int[4], y = new int[4];

        // parte inferiore verde
        x[0] = 0;   y[0] = 50 - y1;
        x[1] = 100; y[1] = 50 - y2;
        x[2] = 100; y[2] = 100;
        x[3] = 0;   y[3] = 100;
        g.setColor(Color.GREEN);
        g.fillPolygon(x, y, 4);

        // parte superiore azzurra
        x[0] = 0;   y[0] = 50 - y1;
        x[1] = 100; y[1] = 50 - y2;
        x[2] = 100; y[2] = 0;
        x[3] = 0;   y[3] = 0;
        g.setColor(Color.CYAN);
        g.fillPolygon(x, y, 4);

        // assi, gradazioni e cerchio
        g.setColor(Color.BLACK);
        g.drawLine(0, 50, 100, 50);
        g.drawLine(50, 0, 50, 100);
        for (int i = 10; i < 100; i += 10)
            g.drawLine(45, i, 55, i);
        g.drawOval(0, 0, 100, 100);
        
        g.drawString(a + "°", 1, 10);
        g.drawString(h + "", 82, 10);
    }

    public int getH() {
        return h;
    }

    public void setH(int h) {
        this.h = h;
        repaint();
    }

    public int getA() {
        return a;
    }

    public void setA(int a) {
        this.a = a;
        repaint();
    }

}

Associare nel Main a jHorizon il gestore di evento MouseMoved:

    private void jHorizonMouseMoved(java.awt.event.MouseEvent evt) {                                    
        int x = evt.getX(), y = evt.getY();
        jHorizon.setA(x - 50);
        jHorizon.setH(50 - y);
    }