Orion, with its characteristic belt and skewed hourglass shape, is one of the most recognizable winter constellations for those of us in the northern hemisphere. Orion rises and sets later and later each night, until eventually we can no longer see it and we know that spring has arrived.
You probably know that Orion’s position in the sky is related to the angle of the Earth’s axis, to its daily revolution, and to its rotation around the sun. You might also know that Orion does not move with respect to the other stars – it is always found next to Taurus and Gemini. We just move into position to see it during the winter months.
So where exactly is Orion? That is a complicated question! For one thing, the seven bright starts that make up the heavenly hunter are not all the same distance from us. The distance from us to Rigel, the star in the southwest corner of Orion, is about three fourths the distance to Betelgeuse, the star in the northeast corner. Because of our point of view here on Earth, they appear to be at the same distance from us on a curved plane in the dome of the sky.
That is the same entirely reasonable model that Egyptians, Indians, Chinese, and Greek astronomers used thousands of years ago. From our observational point on the surface of our planet, it makes so much sense that even modern astronomers continue to speak as if the stars move across the sky.
Before we get into the question of where Orion is, we have to deal with the problematic concept of “up.” We are used to seeing globes on our desks that have the north pole of the Earth at the top, reclining by about 22 and-a-half degrees to represent the Earth’s axial tilt. Similarly, when we look at diagrams of the solar system, the Earth’s north pole is often toward the top of the chart, and we call the top of the sun the solar north pole.1 All this has the effect of giving us the impression that “up” is “north.”
Holding that solar system model in mind, imagine it is noon on the summer solstice, longest day of the year in the northern hemisphere.2 If you are standing on the Tropic of Cancer (it passes just north of Havana, Cuba), which is the exact northern latitude that matches the Earth’s axial tilt, the sun will be directly overhead.3 It’s hard to argue that “directly overhead” is anything other than “up.” However, if you return to that solar system model, you’re actually standing sideways relative to the Earth, with your head pointed toward the sun. The length of your body is aligned with the imaginary disc of Earth’s orbit that is roughly aligned with the sun’s equator.
OK, so how do we get back to that answer of where Orion is? Well, Orion is a winter constellation. That means that we can see it during the night from about November to February. So, we know that it is on the “night” side of Earth during those months. Astronomers mark positions of stars and planets from a coordinate system that is set to 0 in the direction of the sun on the spring equinox – one of two days of the year when we have the same amount of daylight and night. The spring equinox falls on or around the 20th of March each year, so projecting forward 8 months puts Earth on the other side of the sun, right about 220 degrees around its orbit. Since winter is fast approaching at that time of the year, the northern hemisphere is tilted away from the sun. Orion is “south” of the line marked by Earth’s orbit,4 so it would be “below” the planets in that solar system chart we keep referring to.
More generally, we can say that the inhabitants of the northern hemisphere see stars that are south of the planets’ orbits in the winter, and stars north of the planets’ orbits in the summer. So what about the rest of the year? It is a mix as we slowly transition between the longest and shortest days. Right in the middle are the two equinoxes that we just encountered. On those days, roughly 20-21 March and September, every latitude on the Earth gets the exact same amount of daylight and night.5 On either of these days, if you stand right on the equator and look straight up at noon, the sun is right over your head and the length on your body lies along the plane of the Earth’s orbit. Twelve hours later, your feet are pointed directly at the sun and either Virgo (March) or Pisces (September) are directly up. Since the tilt of the Earth is now exactly aligned with the direction of its orbit, the length of your body is once again lying sideways along Earth’s orbital plane.
It is not easy to get these concepts reconciled between the models you see on paper and what you experience when you look up at night. Imagine the amount of effort that it took ancient astronomers to rethink their model of the solar system based on precise measurements and patient observation.
1 The north pole of a star or planet is in fact defined by its revolution. Curl the fingers of your right hand in the direction of rotation. Your thumb points in the direction of its north pole.
2 This is the shortest day of the year for those Down Under.
3 I won’t talk about which timezone you have to be in when the earth passes precisely across the imaginary line marking summer solstice, if you don’t bring up Daylight Savings Time. Agreed?
4 Astronomers call this imaginary line the ecliptic. The twelve constellations that are about level with the earth’s orbit are the zodiac that you might be familiar with from popular culture.
5 Depending on your latitude, you may see light in the sky before sunrise and after sunset due to the Earth’s curvature. This effect is larger near the poles and smaller near the equator. Right on the equator, when the last tiny crescent of sun disappears below the horizon, it gets dark fast!