When does the star Sirius vanish in Egypt and when does it reappear?

Answer 1

Helical setting (sets just before sunset, or when it "vanishes")

The helical setting of Sirius for Egypt is generally in late May. See "Problems in determining helical rise/set dates" below for more information on why an exact date for the helical setting of Sirius is hard to calculate.

"Dog Days"

The time period between the helical setting and the helical rise for Sirius is approximately 70 days.

Helical rise (rises just before dawn, or when it "reappears")

The helical rise of Sirius was considered to be the marking day in which the Nile river would soon annually flood, and also marked as the beginning of the Egyptian new year. Because of this, it is sometimes called the Nile Star. Today, the helical rise of Sirius for Egypt is in early August. See "Problems in determining helical rise/set dates" below for more information on why an exact date for the helical rise of Sirius is hard to calculate.

Problems in determining helical rise/set dates

The exact dates pertaining to the helical rise/setting of Sirius or any other star can be rather hard to calculate for several reasons (light pollution, clouds, latitude/longitude of the observer's location, etc. to name a few). This also greatly depends on where in Egypt. Over time, the dates for both the helical rise and the helical setting have drastically changed over the course of thousands of years and tends to change by 2-3 days over the course of a couple hundred years.

As a general rule of thumb, the further north you are, the later Sirius will "rise" and the further south the earlier it will "rise".