Perihelion is the point in a celestial object's orbit where it comes closest to the Sun. At this position, the object moves at its fastest orbital speed due to the Sun's stronger gravitational pull, following Kepler's laws of planetary motion discovered in the early 1600s.</p><p>Earth reaches perihelion around January 3rd each year, when we're approximately 91.4 million miles (147.1 million kilometers) from the Sun—about 3 million miles closer than at our farthest point in July. Surprisingly, this means Earth is actually nearest the Sun during winter in the Northern Hemisphere! This demonstrates that seasons are caused by our planet's tilted axis, not our distance from the Sun.</p><p>The most dramatic example is Halley's Comet, which speeds up tremendously as it approaches perihelion every 76 years, racing around the Sun at over 120,000 mph before slowing down as it travels back to the outer solar system. Mercury, being closest to the Sun, experiences the most extreme difference between its perihelion speed (35 miles per second) and its slower aphelion speed.</p><p>The term comes from Greek words meaning "near" and "sun." Understanding perihelion helps astronomers predict orbital mechanics, plan spacecraft missions, and explains why comets develop their spectacular tails only when approaching the Sun's intense heat and radiation.