The season also star

Have you ever wondered why stars are visible only at certain times of the year?

Explain why the stars appear only a few certain times

Why, for example, do the Northern Hemisphere night sky observers only see the Hunter of Orion the Hunter in the cold winter months? In the summer, people see Scorpius - Scorpion , not Orion in the southern sky? Spring nights bring the constellation Lion (Leo - the Lion) . Fall is the time of the celestial constellation (Pegasus). This change is quite faint.

If you look at the night sky from dusk to dawn we will notice that some stars grow from the eastern horizon at dusk. These stars move through the sky at night and disappear into the western horizon at dawn. There is no mystery here because the sun itself is similar. The cause of this is the Earth's rotation.

But as time goes by, we will notice something quite confusing.

The low-lying stars on the western horizon in the early hours will disappear from our sight within a few weeks. Their positions were replaced by new constellations a few weeks ago lying high in the sky at sunset. In fact, with time, it seems that all stars gradually move westward while new stars move to the eastern horizon to occupy space.

Constellation Hunter.(Photo: Novaspace.com)

But why does this transition take place?

4 minutes a day.

If we synchronize the clock using the movements of the stars we will discover that the earth will complete a rotation around the shaft not within 24 hours but actually 23 hours 56 minutes . This is a day based on the apparent movements of stars in the sky, which astronomers call the ' astronomical day ' (sidereal), the Latin root of the ' star '.

When this happens, we all move around the sun on an annual journey of about 600 million miles.Earth's orbit is almost a circle. If you observe from the sun, the earth will move about one degree a day, because we take about 365 days to go through a 360 degree circle. If you look from the earth - our vantage point - the sun seems to shift and it changes its position in the sky by about 1 degree a day, when calculated on the background of the stars.

Of course, we cannot see stars during the day but astronomers can measure the position of the sun. The direction of the sun's apparent movement towards the east compared to the stars. Because the sun's daily dive process (due to the rotation of the earth) seems to go west, the sun's movement causes a day to be measured by the sun (the time the earth orbits itself): The earth must rotate (ie about 4 minutes) more than a circle about 1 degree to complete a 24-hour day when measured by the sun.

Scorpion constellation.(Photo: Dkimages.com)

The small conversion every day causes different stars and constellations to appear at different times of the day.The sun changed position slowly, but it was so slow that stars rising at sunset also changed.

If you want to experiment, watch from a location you can return to on a cloudy evening. Notice the exact time a star is directly aligned with an object, such as an electric pole or a roof. Observe the next night, the same position and the star will arrive the previous day 4 minutes earlier than the previous day (of course your watch must be very accurate).

You are observing the impact of earth motion around the sun.

Time measured by stars and time measured by the sun

At this point you may find it difficult to understand. If the earth only takes 23 hours 56 minutes to spin around the axis, why do we say that a 24-hour day?

Astronomers have created a special type of clock that adjusts the time according to the stars.This astronomical clock measures astronomical time. There is no morning or evening for astronomical hours.

Leo.(Photo: Philologos.org)

With the watches we use every day, the hour hand completes 2 times 12 hours in a day. But for astronomical clocks, there are 24 hours of numbers instead of 12 and the hourly clock moves to each number only once a day. Hours are numbered from 00:00 to 23 hours and then start again at 00:00. Another difference between a regular clock and an astronomical clock is an astronomical clock that runs faster than 5 minutes.

If we use astronomical clocks, during the year there will be times when the sun reaches the highest position at noon, but at other times it will be in that position at midnight or dive at 6 am. Of course we are accustomed to waking up in the sun and going to sleep at night, so astronomers have developed an 'intermediate' sun - an imaginary sun and deviant from The real position of the sun in the sky most of the time.

However, it is the intermediate sun that controls our lives and forms the 24-hour timeline that people are so familiar with.

Scorpio.(Photo: Blogger.com)