As seen from the Northern Hemisphere, the stars seem brighter in winter. Why? It’s because – as seen from this hemisphere – we’re actually looking toward many, many more stars in summer than in winter.
In summer, our evening sky is facing toward the center of the Milky Way galaxy – some 25,000 to 28,000 light-years away. We don’t see into the exact center because it’s obscured by galactic dust. But, as we peer in summer edgewise into the galaxy’s disk, the hazy quality of the summer sky is really the combined light of billions of stars in the direction of the galaxy’s center.
In winter, we’re looking the opposite way – into the spiral arm of the galaxy in which our sun resides. The winter stars tend to be closer to us – and there really are some gigantic stars located in this direction. We’re looking edgewise into the disk of the galaxy in winter, too. But we’re looking toward the outskirts of the galaxy – so we’re seeing far fewer stars, and we’re looking more deeply into the space beyond our galaxy’s boundaries. That’s why the winter sky has a clearer, sharper quality than the summer sky.
While the answer is reasonable, I think the person who wrote the original question may have meant something else. For one thing, in many areas, summer evenings are slightly hazy because of moisture in the air, while winter evenings are clearer because cold air can’t hold as much moisture (back when astronomers actually sat in telescope domes, there was a saying that the clearest nights are always the coldest). Also, the summer sky, as noted, has a lot of faint stars, but not as many really bright ones as the winter sky. Over half the brightest stars in the sky are concentrated in a small area called Gould’s Belt, the area between Orion and Auriga, which is most often noticed in the winter sky, when it is up in the early evening.
I think that Stars seem brighter in winter because Earth becomes closer to NCP in winter than summer. look to the Earth’s orbit when perihelion is located at – 23.5 arc degree but Aphelion is located at + 23.5 arc degree to ecliptic plane.
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