Anyone ever experienced unpredictable results when using an OpenGL light that''s perfectly perpendicular to a normal? I''m rendering several rectangular prisms, and I have a light that I''m trying to simulate the sun and moon with like this:

	float afAmbientLight[4];
	if (m_fFactor >= 0.0f && m_fFactor <= 0.5f)
	{
		//sun''s out
		afAmbientLight[0] = 0.1f + 0.9f * sin(3.14159 * m_fFactor * 2.0f);
		afAmbientLight[1] = 0.1f + 0.9f * sin(3.14159 * m_fFactor * 2.0f);
		afAmbientLight[2] = 0.1f + 0.9f * sin(3.14159 * m_fFactor * 2.0f);
		afAmbientLight[3] = 1.0f;
	}
	else
	{
		//moon''s out
		afAmbientLight[0] = 0.1f + 0.1f * sin(3.14159 * (m_fFactor - 0.5f) * 2.0f);
		afAmbientLight[1] = 0.1f + 0.1f * sin(3.14159 * (m_fFactor - 0.5f) * 2.0f);
		afAmbientLight[2] = 0.1f + 0.1f * sin(3.14159 * (m_fFactor - 0.5f) * 2.0f);
		afAmbientLight[3] = 1.0f;
	}
	glLightModelfv(GL_LIGHT_MODEL_AMBIENT, afAmbientLight);

	glLightf(GL_LIGHT0, GL_SPOT_EXPONENT, 1.0f);
	glLightf(GL_LIGHT0, GL_SPOT_CUTOFF, 90.0f);
	glLightf(GL_LIGHT0, GL_CONSTANT_ATTENUATION, 1.0f);
	glLightf(GL_LIGHT0, GL_LINEAR_ATTENUATION, 0.0f);
	glLightf(GL_LIGHT0, GL_QUADRATIC_ATTENUATION, 0.0f);
	//I don''t know why this works, but I have better luck
	//getting the sun or moon effect I want if I let the
	//global ambient take care of ambient and let the
	//directional light take care of diffuse and specular
	float fAmbient[4] = {0.0f, 0.0f, 0.0f, 0.0f};
	glLightfv(GL_LIGHT0, GL_AMBIENT, fAmbient);
	float fDifSpec[4];
	if (m_fFactor >= 0.0f && m_fFactor <= 0.5f)
	{
		fDifSpec[0] = 1.0f;
		fDifSpec[1] = 1.0f;
		fDifSpec[2] = 0.0f;
		fDifSpec[3] = 1.0f;
	}
	else
	{
		fDifSpec[0] = 1.0f;
		fDifSpec[1] = 1.0f;
		fDifSpec[2] = 1.0f;
		fDifSpec[3] = 1.0f;
	}
	glLightfv(GL_LIGHT0, GL_DIFFUSE, fDifSpec);
	glLightfv(GL_LIGHT0, GL_SPECULAR, fDifSpec);
	//for all practical purposes, a directional light
	//shines in the direction opposite its position
	//and spot direction is ignored
	float fPosition[4];
	if (m_fFactor >= 0.0f && m_fFactor <= 0.5f)
	{
		fPosition[0] = (float)cos(m_fFactor * 2 * 3.14159);
		fPosition[1] = (float)sin(m_fFactor * 2 * 3.14159);
		fPosition[2] = 0.0f;
		fPosition[3] = 0.0f;
	}
	else
	{
		fPosition[0] = (float)cos((m_fFactor - 0.5f) * 2 * 3.14159);
		fPosition[1] = (float)sin((m_fFactor - 0.5f) * 2 * 3.14159);
		fPosition[2] = 0.0f;
		fPosition[3] = 0.0f;
	}
	glLightfv(GL_LIGHT0, GL_POSITION, fPosition);
 

The normals on the prism sides facing front and back are (0, 0, 1) and (0, 0, -1). What I''m seeing is these two sides "flicker" - and they flicker at exactly the same time each "cycle". Are there certain angles that light up the surface? Love means nothing to a tennis player

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