Question: I beg your pardon for such a silly question, but I've never really understood the explanation they gave in the Geography textbooks. If the slanting Sun's rays due to the Earth's axial tilt can cause such a huge temperature difference, why isn't all our ocean get frozen when the Earth is farthest from the Sun (in the Earth's orbit)? 
Earth revolves around the Sun in an elliptical orbit with a mean eccentricity of 0.0167. As a result, Earth's distance from the Sun (center-to-center) varies with mean values of 0.9832899 AU (147,098,074 km) at perihelion (closest) to 1.0167103 AU (152,097,701 km) at aphelion (most distant). If the slanting solar rays can freeze water at the poles, this difference should cause the entire water body to freeze, isn't it? I'm definitely missing something here. Can anyone please help me?

Answer: 
If you compare the Earth-Sun distance in the summer and in the winter, you will notice that it differs by only about 4%. Given that the radiation strength of the Sun is proportional to 1/distance^2, the radiation intensity is roughly 8% STRONGER in the WINTER (on the northern hemisphere) than in the summer. 

But the distance to the Sun is not the only factor we should take into account. In the winter, the Sun doesn't shine vertically on the ground. If the angle between the vertical and the sun is, let's say, 45 degrees, then the intensity will be weaker by a factor of sin(45 degrees)≈0.71. 

Conclusion: The distance to the Sun is less important than the angle at which the light reaches the surface. Since the angle is very small at both poles over the year, they are both extremely cold. 

Additional: why is the Summer on the Northern hemisphere colder than on the southern hemisphere? Although the angles at which the light reaches the surface is almost the same, now we can take into account the distance to the sun. During the European Summer, the distance to the Sun is 4% larger than during the Australian Summer. Therefore, the European summer is a little colder.
It is a very general doubt and really important.

Slanting sun rays coming to earth from sun are refracted and rarely reach earth. They deviate by refraction.

At equator and tropics refraction is less.
As you know for $\theta = 0$ no refraction take place and as angle increase refraction increase.

As poles do not get much sun rays throughout year and centuries , they remain cold.