The sun never changes its temperature in relation to Earth’s seasons. Since it is literally a ball of fire facilitated by nuclear reactions, its surface stays hot all year round. The extremely high heat of the sun causes it to emit copious amounts of radiation in all directs from its spherical shape. Since the sun emits light rays in our visible spectrum, it also emits rays in the form of infrared waves as well as ultraviolet waves. This radiation travels throughout the solar system and, due to the way that light waves carry energy through momentum, warms everything that it comes into contact with.
So, if the sun does not change in temperature throughout our Earthen year, how is it colder in winter (for the Northern hemisphere)? This can be explained by the configuration and position of the Earth itself as opposed to anything to do with the sun, which is in effect a constant.
The Earth has colder areas in Winter because of the tilt caused by our axis. This tilt means that the sunlight that reaches Earth is spread over a larger area in winter (for the areas where it is cold in winter), and more spread out light means that the light is weaker in these regions. Weaker light means less energy per unit area and therefore a colder area of Earth.
The Earth being colder in Winter for the northern hemisphere is nothing to do with its distance from the sun. In fact, the northern hemisphere is furthest from the sun when it is in Summer thanks to the tilt of Earth.
But because this distance does not make a huge overall difference to the distance from the sun, the deciding factor is still the tilt of the Earth on its axis.
If you were to plot the Earth’s orbit around the sun in a year, it would appear completely circular, but there would be a change when the Earth is going through the northern hemisphere’s Summer.
But since this distance change is miniscule (in terms of space distances), you cannot even notice a distance with the naked eye.
This tilt of Earth that we are talking about is the 23-degree tilt relative to a straight line. The line that goes from North to South pole, or the axis, is 23-degrees tilted away from its respective orbit around the sun. So, the northern hemisphere’s Wintertime sees it tilting away from the sun.
This is where the phrase ‘low Winter sun’ comes from, because of the Earth’s tilt, the sun appears lower in the sky in Winter for the northern hemisphere. Therefore, a Winter sun’s light is spread more across the same area of hemisphere and feels colder thanks to the light’s energy and warmth also being spread thinly.
The difference in hemispheres is also important to consider. Although the question is about the sun being ‘colder’ in Winter, this is only specific to the northern hemisphere.
When the Earth is tilted towards the sun with the northern hemisphere, this is the areas that experiences Summer. But when the northern hemisphere experiences Summer, the southern one is experiencing Winter. This is because the southern hemisphere is tilted on the axis away from the sun, and the same effects occur: seeing the sun’s rays being more thinly spread across the region of land, therefore being colder.
These differences are obviously more obvious at the extremes, the north and south poles. In a northern hemisphere winter, the sun will be seen less in the sky at the North pole and more at the South pole. So, it follows that areas of land at the equator are less effected by the tilt of the Earth and see the sun for a very similar amount whether it is Summer or Winter.
Because of all of this, the question posed of why the sun is colder in Winter, is a very northern-hemisphere-centric one. Although it is also cold in Winter for the southern hemisphere, it depends both how far the land is from the extremities of the globe, and also how close it is to the equator. Since these areas are either more effected or not effected at all by the tilt of the Earth on its axis.
If we take North America, an area of land in the northern hemisphere, we can see that it is far from the equator compared to Colombia. Because of this, the Winter in North America is significantly colder than that in Colombia.
This is thanks to the North America being more greatly affected by the tilt of the Earth on its axis and the way that the sun’s rays have to travel at an angle to reach some of the land. The same amount of light will reach the northern hemisphere, but they are more spread out and therefore carry less energy per unit area; this causes the sun to feel colder in Winter.
Although the sun’s temperature does not change on its surface, or the radiation it sends out, the tilt of the Earth is the deciding factor when it comes to how long the sun stay in the sky in each season, as well as how much further the light is spread across each area of land in Winter.
To conclude: the sun is not itself cold in Winter, but the tilt of the Earth determines how far the rays will spread and therefore the energy and warmth that comes with them.