Tuesday, August 2, 2011

The critical concept of heliacal rising

Above is a simple sketch illustrating the important concept of heliacal rise.

We've discussed in a previous post the fact that stars along the ecliptic rise about four minutes earlier each night. The reason for this can be seen in the diagram, which shows the earth going around the sun in the direction indicated by the arrow along the orbital path (to the right of the diagram, from the position earth is depicted). The progress of the earth each day along this orbital path is what causes the stars to rise four minutes earlier each passing day.

We've also discussed the fact that some stars, by virtue of being located "on the ceiling" of the hypothetical "room" in which the earth orbits the sun, will be visible from the northern hemisphere each night (stars "on the floor" would be visible each night for observers in the southern hemisphere). Stars "on the walls," however will spend some part of the year obscured by the sun as earth orbits to a position in which the sun is between earth and "their wall."

For example, in the diagram above, if the earth were further back on its orbit (back where it was at the beginning of July, for example), you can see that observers on earth would be facing the position of the stars in Orion during the daytime. However, as earth continues along its orbit, there will be a point at which the rotation of the earth will reveal the stars of Orion in the sky before the continued rotation reveals the rising sun.

The earth has now reached that location, such that Orion in all his majesty can be seen in the eastern sky prior to the sunrise. At a location in the northern hemisphere of about 35o north latitude, morning twilight currently begins around 5:45 and the sun rises at about twelve minutes after 6 am (getting a minute or so later each day, as we move further away from summer solstice and closer to equinox).

At the same latitude, the stars of Orion begin to come into view during the half hour prior to 4am, as the earth turns as indicated by the curved arrow above, bringing the stars up over the rim of the eastern horizon (actually, the eastern horizon is plunging "downwards" as earth spins, allowing an observer to see those stars). The eastern side of the sky is noticeably lighter and bluer than the western half as sunrise approaches, creating the impressive sight of the stars of Orion rising in the beautiful light-blue east.

The first return of a star on the eastern horizon just prior to sunrise, after its annual period of absence due to the interference of the sun, is known as the heliacal rising. The heliacal rising of the zodiac constellations, as well as of the stars of Orion and especially Sirius trailing Orion, were extremely important to ancient civilizations and their descendents around the world (including those who retained strong influence of those ancient civilizations into recent times, such as the Polynesians and the Indians of the Americas).

The earth has not yet reached the point on its orbit when the heliacal rising of Sirius, the brightest star in the sky (other than the sun), will take place, but earth is on its way to that spot, reaching it on August 7. Right now, because Sirius is to the "left" of Orion (in the orientation of the sketch above), it is still obscured by the sun when the earth turns in its direction, but as earth progresses around its track, you can visualize why Sirius will soon become visible in the light-blue east and will then begin "moving" towards the darker and darker part of the sky before sunrise on consecutive mornings as we go around.

The diagram and discussion above should help you gain a clear understanding of the critical concept of heliacal rising.