1. Why Is It Cold in January When Earth Is Closest to the Sun?
Earth orbits the Sun once a year. This path is called the orbit. The point closest to the Sun is the perihelion, and the point farthest away is the aphelion. In 2026, the perihelion occurred around January 4, and the aphelion around July 7 (Japan time). [7] Since January is deep winter in the Northern Hemisphere, it is cold even though Earth is close to the Sun. NASA explains that the fact that perihelion is in January and aphelion is in July is evidence that distance from the Sun is not the cause of the seasons. [1] The main cause of seasons is that Earth’s axis of rotation is tilted 23.4 degrees relative to its orbital plane. [2]
The difference in distance between perihelion and aphelion is small. The distance at perihelion is about 147.1 million km, and at aphelion, it is about 152.1 million km. The difference is about 5 million km, which is only about 3.3% of the aphelion distance. [6] NASA shows figures of about 91.4 million miles and about 94.5 million miles, which are roughly the same ratio. [1] If you scale Earth’s orbit to a circle with a radius of 15 cm, the difference between the distance to perihelion and aphelion is about 5 mm. Earth’s orbit is nearly circular. So, does this 3% difference have no effect? [6]
2. Calendar Dates Show the Spring-to-Autumn Half-Year Is Longer
According to the National Astronomical Observatory of Japan’s 2026 calendar, the spring equinox was March 20 at 23:46, the summer solstice was June 21 at 17:25, the autumn equinox was September 23 at 09:05, and the winter solstice was December 22 at 05:50 (Japan time). [3] From these times, we can calculate the length of the seasons. From the spring equinox to the summer solstice is about 92.7 days. From the summer solstice to the autumn equinox is about 93.7 days. From the autumn equinox to the winter solstice is about 89.9 days. [3] Summer is about 3.8 days longer than autumn.
Which half-year is longer than half a year? The time elapsed from the spring equinox to the autumn equinox adds up to about 186.4 days. A year is 365.25 days, [2] so half a year is about 182.6 days. This means the half-year from spring to autumn is about 4 days longer than half a year. The National Astronomical Observatory’s calendar wiki also explains this summer half-year as about 186 days. [4]
The reason is that orbital speed varies. Earth orbits faster near perihelion and slower near aphelion. Therefore, the half-year from spring equinox to autumn equinox, which includes the aphelion in July, involves passing through the section where Earth orbits slowly. [4] Winter, when Earth is closer to the Sun, is shorter because Earth rushes through that part of the orbit. This difference in length applies to the Northern Hemisphere; in the Southern Hemisphere, it is the opposite. [4]
3. Earth Moves Faster Near the Sun, Where Sunlight Is About 7% Stronger
The average orbital speed is about 29.8 km per second. It is slightly faster near perihelion and slightly slower near aphelion. Because there is a relationship where Earth is pulled more strongly and runs faster when closer to the Sun, the difference in speed is small because the difference in distance is small. [4] This difference in speed leads to the difference in season lengths counted in the previous section. The half-year from spring to autumn, passing slowly on the aphelion side, takes more days.
The difference in distance also shows up in the strength of sunlight. The intensity of sunlight is inversely proportional to the square of the distance. Comparing aphelion and perihelion, the ratio is (152.1 ÷ 147.1) squared, which is about 1.07. This means sunlight is about 7% stronger at perihelion. Space.com also cites NASA officials stating there is about a 7% difference. [9] However, this is a ratio of sunlight intensity, not a difference in temperature. [9]
4. How the Date of Closest Approach to the Sun Shifts Over About 20,000 Years
In 2026, perihelion was on January 4, and the next perihelion is on January 3, 2027. [7] The dates when Earth passes perihelion and aphelion are not the same every year; they can shift by up to 3 days. The National Astronomical Observatory explains that the orbit changes due to the influence of the Moon and planets, causing the passage times to shift each year. [5] [6]
Looking at a longer timescale, the season in which perihelion occurs is also moving. NASA explains that the combined cycle of the change in the direction of the axis (precession) and the change in the direction of the orbit averages about 23,000 years. [8] Depending on the source, this range is given as 21,000 to 26,000 years, so it is good to think of it as a cycle of "a little over 20,000 years." Half of this cycle is about 10,000 years. From this cycle, it is thought that there will be a time when perihelion occurs in July. By the same logic, at that time, the Northern Hemisphere’s summer half-year should be shorter. [8] (This is a prediction based on the explanation in this article.)
5. Slow Changes in Earth's Orbit Shape and Axis Tilt
It is not just the position of perihelion that changes. According to NASA, the shape of the orbit (eccentricity) changes with a cycle of about 100,000 years, and we are currently on the way to becoming more circular. [8] This means the distance difference of about 3% is also gradually getting smaller.
The tilt of the axis also changes. Over the past 1 million years, it has moved in a range of 22.1 to 24.5 degrees with a cycle of about 41,000 years, and it is currently slowly decreasing. [8] When these changes overlap, the amount of solar radiation reaching middle latitudes can change by up to 25%, according to NASA. [8] The numbers we remember, like "nearly circular" and "23.4 degrees," are values for the current era within these long changes.
6. Count the Days of the Spring-to-Autumn Half-Year Yourself with a Calendar
What you can do today: Look at a calendar and count the days between the spring equinox (March 20) and the autumn equinox (September 23) in 2026. Adding up the days month by month gives 187 days, which is longer than half a year (about 182.6 days). You can check it for yourself. Since you are counting only by dates, it will be slightly different from the article’s calculation of about 186.4 days. [3]
Another activity: Draw a circle with a radius of 15 cm and mark a point 5 mm off-center as the "Sun." You can see how small the difference in distance between perihelion and aphelion is. For official information, the National Astronomical Observatory’s calendar for Reiwa 8 (2026) lists the times for the equinoxes and solstices. [3] It would be interesting to look up the times for the autumn and spring equinoxes in the 2027 calendar and calculate the length of the winter half-year using the same method to compare it with the summer half-year. [3]