The fact is that the sky above us is changing during the year. At different times during the year we can see some constellations on the sky and the deep-sky objects they contain and some not, and also the Moon and the planets sometimes can be seen, and sometimes not.
Lets start with the constellations and the stars and the so-called deep-sky celestial objects that appear on the sky inside the borders of the constellations. First of all the stars and that objects are so far away that we cannot observe any dynamical changes, either on their surface or either in their relative position and movement to each even over very long period of time. But the stars and constellations are not still on the sky, during one night we can see some constellations rise on the east sky, while some move from east towards the west and some are setting bellow the west horizon. That happens because of the Earth's rotation around its own axis of rotation with a cycle of 24 hours (or 23 hours and 56 minutes exactly). The sky is changing also during a longer period of time with 365 days cycle, because of the Earth's revolution around the Sun which is 365 days. For example, in spring at the zenith high on the sky we can see some constellations, but in the summer those constellations are no longer in the zenith, but they are close to the northwest horizon and ready to set. Now in the summer at the zenith we can see different constellations and stars. The same applies for the fall and the winter. That is why the constellations that are in the best position for observing (usually when they are at or close to the zenith or when they reach its highest point in the sky above the south horizon) in spring we call them ''spring constellations'', that ones the best visible in summer we call “'summer constellations'', and those which are the best visible in the fall we call ''fall constellations'' and in winter we call ''winter constellations''. So, during the year, non-stop new constellations rise on the east every new month, bringing the objects they contain in good position for observing.
That means, if you came once at the observing event, for example in spring, you still did not see the whole sky and all that interesting stars, constellations, nebulae, star clusters and galaxies, but only small part of all that interesting objects, which are visible on the sky in spring, but you have missed the summer, the fall and the winter constellations and all the objects they contain.
The above mentioned also applies for the Sun, the Moon and the planets too, with different time cycle. OK, the Sun we can observe every day, but the Moon is not visible every night, and also the planets are not visible every night on the sky. That means if you come only once on the observing events you will see some planets, but some of them not, and maybe or not the Moon.
Also, the Sun, the Moon and the planets are closer objects than the deep-sky celestial objects, so we can observe some changes of dynamical nature, either on their surface or because of their movement relative to each other.
For example, the Sun always have different look, its sunspots appear and disappear non-stop, sometimes in smaller number, sometimes in bigger number. Sun's surface is not solid, but is dynamic and always appears different, always we can see some interesting surface details with particular shape and size, and not to mention the Sun's prominences, which non-stop appear and disappear and always have different shape and size, and are easiest to see when they are over the Sun's edge.
Coming once at the observing events and seeing the Moon does not mean that you have seen everything possible and available on the Moon's surface. The Moon changes its phase in a cycle of 28 days, because of its rotation around the Earth and its disc is illuminated more or less when seen from the Earth. Moon's illumination, or also called phase, can be from zero percent when is young Moon, its phase (or illumination) can be 50% and then the Moon is at its first quarter or last quarter, and can be 100% , and then the Moon is full. Because of these changes, different craters and surface features sometimes are in good position for observing (when the Sun's light is hitting them with a big angle) and sometimes are in not good position for observing (when the Sun's light is hitting them directly from above). The craters and other surface details are best seen when they are at the border between the illuminated and the non-illuminated part of the Moon. That border is called Lunar terminator. At the Lunar terminator the angle at which the craters are illuminated from the Sun's light is 90 degrees, and the craters are part in shadow and part illuminated and not in shadow, so it is very easy to get a perception of the depth of the craters and the other surface features. Because the Moon changes its phase every night, that means the terminator every night moves a bit away from where it was the previous night, placing other craters and surface features in their best position to be seen from the Earth.
Also the Moon distance from the Earth is changing, from about 360,000 km to about 400,000 km. When is closer, it appears a bit bigger on the sky and also in the telescope.
Also the dynamic change nature applies for the planets because they are relatively close to the Earth and we can observe some dynamic changes on their surface or changes in their relative position between each other or change in the position between the planets and their own moons. For example Jupiter's moons can appear from only one side or another side of the planet, or some can be from one side and some from the other side, or some temporarily can not be seen because they are behind the planet (they are eclipsed by the planet) or one or more moons can be in front of the Jupiter's disc or they can cast their shadows on Jupiter's disc which shadows will be visible like small black dots on Jupiter's surface. Also Jupiter's Great Red Spot, a big storm system several centuries old can be observed for three hours and after is not visible for the next seven hours because of Jupiter's rotation which lasts about ten hours. Jupiter's big four moons sometimes appear apparently very close to each other, even it looks like they are touching each other, and sometimes they eclipse each other. Also over longer period of time can be noticed some small change in the shape of Jupiter's bands and clouds.
Mercury and Venus show phases like our Moon from 0 when their disc is 0% illuminated to 1 when their disc is 100% illuminated because of their rotation around the Sun and because their orbits are closer to the Sun than our planet's orbit, so sometimes they appear crescent, and sometimes they appear fully illuminated in the telescope and all in between. At the same time during the change of their phase, they also change their apparent size which can be observed with the telescope. Change in their size happens because the distance between the planets and the Earth changes during their rotation around the Sun, and at their closest to the Earth they disc will appear five to six times bigger.
Mars' disc changes its phase between 0.85 and 1, and when is at its closest to the Earth, its disc also appears about five to six times bigger.
Saturn's rings are not always the same when viewed from the Earth because of the Saturn's rotation around the Sun, the tilt of the rings relatively to our view is different, it can be more or less. When the tilt is minimum the rings will almost disappear, and when the rings are maximum tilted then they are in their best position for observing and appear the most beautiful. The time period of the cycle is about 15-16 years.
Also, all planets cannot be seen at once on the sky, that means if you attend only one observing event you have seen some planets, but some not. Sometimes is possible to see two or even three or more planets at once on the sky, but if you want to see all planets, you will need to attend several observing events during the different time of the year.
Sometimes planets can come apparently close on the sky, even very close we can see both of them in the telescope field of view, and that is exciting event and view not to be missed.
So, the short conclusion is that non-stop on the sky we can see some interesting things which we have not seen before. First, simply because during different nights different parts of the sky are available for us for observing (in the summer we can observe the summer constellations and their deep-sky objects, in fall the fall constellations and their deep-sky objects and so on. Second, because non stop some dynamic changes are happening on the surface of the objects (the Sun, Jupiter) or because of the movement of the objects (different Moon's craters can be seen at their best position for observing, the moons of Jupiter change its position, two or more planets can come apparently very close to each other and so on). And third, also non-stop on the sky something new is happening, for example some comet appearance, Sun's and Moon's eclipses, the Moon can eclipse some star or planet, nova event, supernova event and so on.
Also if Sky Observing adds some new astronomical equipment at its events in the future, that is great reason to attend the observing events again. For example, telescope with bigger mirror means that we can see the celestial objects – star clusters, nebulae and galaxies brighter and bigger, and with more visible details. Using telescope with bigger mirror is especially useful when observing the Sun, the Moon and the planets, because it has better resolution power than smaller telescope, and more surface details will be visible, with more contrast and sharper. Also bigger telescope can split the double and multiple stars better than smaller because of its better resolution power. Also telescope with larger mirror collects more light than telescope with smaller mirror and can detect dimmer objects on the sky, and can reach deeper into the universe and we can add more interesting objects on the observing list that we will not be able to see with telescope with smaller mirror.
Reason for coming again can be if some special event has happened or will happen on the sky, like comet appearance, nova or supernova event etc, Sun's or Moon's eclipses, transit of some planet in front of the Sun, or when the Moon eclipses some planet on the sky and so on.
And last, but not least important – sometimes can happen the atmosphere to be extraordinary stable – that means with no any turbulences – and will give us the most beautiful views of the sky we have ever seen – objects we see in the telescope will not appear turbulent but still. This applies especially for the Moon and planets observing, because we want to see details on their surface and also to see them with great contrast and as sharp as possible, and when such night happen, we can also apply very high magnifications, even 1000 times and still the planets will not appear turbulent in the telescope view and we will be able to see much more details on their surface and also the views will be with high contrast and sharp.
Also, the atmosphere some evenings can happen to be very clear, that means we can see more stars on the sky and if we are at dark location far from the city we can see the Milky Way on the sky very bright. On evenings like this the views of the star clusters, nebulae and the galaxies through the telescope will be incredible, and these objects will appear brighter in the view of the telescope and more details will be visible when compared with ordinary night.
All above mentioned should be great invitation and reason for you to decide to come again at the observing events. Just check the page “THIS MONTH: OBSERVING SCHEDULE AND WHAT IS VISIBLE ON THE SKY”, and the NEWS page for any important information and special event announcements and if there is something you haven't seen yet, and like to see it, you can register again or come directly if is announced in advance that still several spots are available for that observing event.