Day observation

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Venus sickle by day, only 17 ° east of the sun (photo through the reversing telescope, 80x), on the left an approaching thundercloud.

Day observation is called in astronomy , but also in geodesy, the observation of stars in the bright day sky. It is a matter of course when observing the sun and not uncommon for the moon , but it is also used for other celestial bodies, in particular for the two inner planets Venus and Mercury , which are always close to the sun. Details on the very bright surfaces of these planets are easier to see in the daytime than at night, and they are also higher up in the sky.

Some large comets can also be observed in the daytime sky - even better on the orbital sections near the sun than at night, where they are only just above the horizon. When they reach the brightness of Venus, they can even be spotted with the naked eye at times.

Sometimes bright fixed stars are also measured during the day, e.g. B. the Pole Star for precise measurements or bright double stars that appear strongly outshone at night. This also includes special satellite observations .

Daytime observations have numerous disadvantages over nighttime observations , but they also have some advantages. The clearer the air and the deeper the sky blue, the better the contrast and observation conditions. The sky becomes darker with increasing sea ​​level , as is easy to see in high mountains and when traveling by air.

Celestial objects and instruments for daytime observations

Shortly before an occultation of Venus by the waning moon, at noon on December 7, 2015

The Venus , after the sun and the moon, the brightest star can often have a day freiäugig see if the angular distance from the sun (or the approximate direction of the sky) is known and more is 20 °. The maps of the planets Venus and Mercury near the Sun, made on large telescopes between around 1850 and 1960, have only been observed in the daytime sky .

Other bright planets (Mars, Jupiter, Saturn) and first-size stars can be seen in good binoculars during the day , but stars of the 2nd to 5th magnitude can only be seen in a telescope of the appropriate size (see picture below, 20 cm reflector telescope ).

However, the telescope or binoculars should have a small exit pupil (maximum 4 mm) because the pupil of the eye is narrower in daylight than at night. This is why pocket binoculars such as Trinovid 8 × 20 or Optolyth 10 × 25 are quite suitable, although they are too faint for night observation.

Mirror telescope C8 , aimed at Venus (40 ° to the right of the sun)

The sun has been used for time and place determination as well as for navigation at sea since ancient times . With a modern theodolite you can also use the Pole Star during the day for precise directional measurements, which, like the sun, is used for astronomical orientation by isolated surveying networks. B. to align high-voltage pylons in the forest or guns.

The rapid development of electro-optical sensors will soon make it possible to carry out automatic daytime observations with star sensors - as has long been the case in manned space travel . The problem of strong scattered light (sky blue) can be dealt with on the one hand with digital filter techniques, on the other hand with optical means such as the polarization filter .

Advantages and disadvantages

Problems with observation in daylight are accepted because of the following advantages:

  • the ability to observe without time restrictions
  • no nighttime tiredness
  • the elimination of expensive overtime , avoidable travel times or night surcharges
  • Contrast reduction or less overexposure (sometimes desirable with Venus or the moon )
  • no lighting necessary, easier manipulation with additional devices
  • less annoying flying insects and mosquitoes .
The (pale) moon over the Valais Alps

On the other hand, there are disadvantages:

Day observation of fixed stars

Stars - d. H. distant suns - are relatively easy to observe in the daytime sky , though

  • the exact "apparent location" on the celestial sphere is known (azimuth and elevation angle),
  • the focus of the optics (the focus at infinity ) is correct - which must be checked carefully beforehand,
  • and the star brightness is sufficient to stand out from the sky blue . This succeeds the more likely the further the star is from the sun; at least it must be 20 °.

Summary: Stars of the first and second magnitude can always be found when the sky is clear, usually also when there are veiled clouds or contrails. When the sky is deep blue, you can also see 3rd to 4th magnitude stars in the eight -inch lens (20 cm lens or reflecting telescope) .

Area versus point brightness

The fact that the latter is possible has to do with an important feature of telescope observations: the higher the magnification ( using a short focal length eyepiece ), the darker the sky appears in the field of view .

On the other hand, the star remains punctiform even at higher magnifications ( apart from lens defects and unsteady air ) until the limited resolution of the device becomes visible with the magnification . Therefore, the star brightness in the telescope remains the same in a certain magnification range, while the sky can be "darkened" by increasing the magnification. The only limiting factor is the side light that catches the eye from the bright surroundings .

Pole Star

An important application of this is the measurement of the pole star in geodesy . The Polarstern can be used advantageously for the precise orientation of surveying networks or instruments - see Azimut Polaris - and if you can do it during the day, effort and costs are lower.

A second magnitude star like Polaris (or one of the Big Dipper ) can be clearly seen in a theodolite at 30x magnification when it is roughly in the middle of the field of view . In order to achieve this, a rule of thumb that includes three parameters is sufficient : latitude , time and date.

With more experience you can find Polaris without calculating: its elevation angle is equal to the width with an error of ± 0.7 ° (so-called pole height, e.g. for Munich and Vienna B = 48.2 °, i.e. elevation angle 47.5 up to 48.9 °). The field of view is usually 2 °, so that the star can soon be found by drawing 3–4 horizontal search loops.

Other bright stars

The same applies to other fixed stars, but there is no avoiding a more precise calculation. A previously well-proven method is to set a star with the same declination in the telescope the night before the desired daytime observation and to fix the mount in this direction. The time difference between the two star passages corresponds exactly to the difference in the right ascension (the star coordinate that corresponds to the geographical longitude ).

The deeper the sky blue , the better the stars stand out. A star 1. Size is almost always visible with a four to eight-inch monitor in the city as soon as it in the visual field of the telescope has and the angular distance from the sun exceeds 20 °. Of course, you need an approximate forecast, e.g. B. Azimuth and elevation angle. Stars of 1.5 mag (e.g. Deneb 1.4 m ) can still be seen near the target axis, while stars of the 2nd magnitude (such as Big Dipper or Pole Star ) have to be set quite precisely. In deep blue skies, third and fourth size stars are also possible, but in city skies usually only 2.0 to 2.5 m . However, from sunset everything becomes much easier. The Venus is one (-4 m ) However, once it is place in the sky to some degree known, and their distance is from the sun at least 15 °. This makes it a good aid for setting the direction and precise focusing .

Some reasons to observe fixed stars during the day can be:

Double stars

The observation or measurement of bright double stars is particularly attractive during the day . In the nocturnal telescope, stars of the first to third magnitude are so bright that their point-like shape inflates strongly and outshines a close companion . In the daytime sky, on the other hand, the same stars appear as fine dots. If the weaker star is brighter than 5th magnitude , it can still be recognized because of the brighter main star, because the eye “knows” exactly where it can be found on the blue surface of the sky. When the air is calm, this technique allows binary stars to be separated up to the theoretical resolution .

Day observation of planets and comets

A classic observation method to determine the astronomical unit is the - admittedly very rare - Venus passages in front of the sun. As a special case, they are not part of the article topic, but should be mentioned briefly. Venus transits only take place twice in approx. 120 years, most recently in June 2004 and 2012. The measurement of Venus takes place relative to the edge of the sun , as the series of images shows.

Sun and venus. Venus transit from June 8, 2004, series of images with 4 ″ Maksutov telescope and ND 4 filter film

Venus and Mercury

Venus in the C8 reflector telescope, still a bit blurred (should look like a small crescent moon)
Close to the sunlit telescope eyepiece (50x). Venus is already visible as a small point in the lower right corner of the sky.

In general, daytime observation of planets requires sufficient contrast in the telescope and with the sky, because they do not appear point-like like stars, but as small disks or in a narrow sickle shape. In the case of the planets Mercury, Venus and Mars, the necessary contrast is usually given because they orbit relatively close to the sun and therefore have a brightly lit surface.

A bright telescope that guarantees sufficient surface brightness even at medium magnification (preferably 50 to 100 times) is favorable . Short focal length reflector telescopes or an upgraded comet finder are well suited .

In the case of Venus, the contrast is always sufficient - even with light cloudiness and in normal binoculars . When Mercury 's visibility is highly dependent on its phase and brightness; 2–3 weeks before or after the proximity to the earth ( conjunction ) the Mercury sickle is large, but usually too faint.

At larger telescopes, daytime observations of the two planets were the only way to explore them optically until space travel began . Because at a maximum of 20 ° or 45 ° angular distance from the sun, they could otherwise only be observed at dusk (and therefore at a low height above the horizon). As many publications and reports around the turn of the century show, the daytime and early twilight observations of the line structures , spots and shadows on the two inner planets have contributed a lot to their exploration, especially by Giovanni Schiaparelli and his student Percival Lowell . In the 20th century u. a. the French A. Dollfus and Eugène Antoniadi rendered outstanding services to these celestial bodies, as well as to the mapping of Mars up to the first space probes of the 1960s.

Outer planets

Whether planets further away from the Sun such as Mars , Jupiter and Saturn are clearly visible in a suitable telescope depends geometrically on their orbit (especially the distance to the Sun). As far as the meteorological conditions are concerned, the brightness of the sky and the transparency of the air are decisive, and to a lesser extent also the unrest in the air . The latter is usually best in the early hours of the morning and in the afternoon when the air temperature changes the least.

However, the outermost planets Uranus and Neptune already receive too little sunlight to stand out in the blue of the sky . In good weather conditions, the limit for a just sufficient brightness contrast is around Saturn , while there are hardly any problems with Jupiter . It is more noticeable due to its reddish color tones and its surface brightness exceeds that of the daytime sky, even with thin cirrus clouds . From about an eight-inch (with good lens telescopes from an aperture of about 6 inches ) even 3–4 equatorial stripes of Jupiter's atmosphere can be made out. When the air is particularly clear, the four bright moons of Jupiter and their charming constellations and eclipses come into the range of visibility.

However, the sky blue causes fine details of the planet's surfaces to become blurred - especially if they do not contain any red tones. The well-known experience of how “pale” the moon looks in the daytime sky gives an impression of this (see picture above). Mercury, Venus and Mars can be easily observed due to their great surface brightness, but with Jupiter the thinner of the 4–6 equatorial stripes mostly disappear in the sky blue.

In calm air conditions z. B. the size, flattening and rotation of the planets, and also the observation of precalculated star occultations can succeed. In spite of its small size, details around the Mars opposition can even be seen on the “Red Planet”, at least the two darkest equatorial stripes on Jupiter and the Cassini division of its rings on Saturn .

Photographic observations are much more difficult than visual observations, especially with the more distant planets. The air turbulence and the risk of blurring can be reduced by taking a series of photos (with digital cameras, for example, with the "sports function") or by using webcams and subsequently superimposing them. The lack of photographic contrast can be improved somewhat through image processing .

Bright comets

Some of the large comets can also be observed in the daytime sky, especially on the orbits near the Sun. Then the observation conditions can be even better than at dusk , when they are only just above the horizon. Such particularly bright tail stars come into the inner solar system every few years.

When they reach the brightness of Venus and are at least 10 ° away from the sun, they can even be seen in the sky with the naked eye for a time.

Day observation from satellites

For artificial earth satellites - especially in low orbits - dusk has always been the best observation time, because they end up in the earth's shadow during the night . Traditional daytime observations, however, are extremely difficult. With larger optics and suitable CCD sensors, however, they can be successful if the sky is additionally attenuated with a polarization filter and very precise path elements are present.

In the first two decades of space travel , within the framework of the Moonwatch organization (supervised by the SAO in Massachusetts), some 100 observation teams worldwide have specialized in observing low-flying satellites at dawn and dusk . Especially important this was to calculate the decays , the re-entries (reentry) of satellites in the Earth's atmosphere just before they burn up. These visual observations, which can be accurate up to 10 ″ despite the rapid movement of the satellites, became superfluous around 1975 thanks to automatic radio processes.

Attempts have also been made to operate Satellite Laser Ranging (SLR) during the day since around 1980 . A well-controlled laser beam hits the satellite even during the day, but triggering the time interval counter requires a sufficient number of reflected light quanta . In the early days of satellite geodesy , this was far too small and drowned in the blue of the sky. Today, measurements of this kind are successful as standard if gate time switching, laser control and reception filtering are well coordinated. This makes it possible in principle to determine the orbit of the laser satellites around the clock.

Occasionally there is also the possibility of observing so-called iridium flares in the daytime , if they are precisely calculated in advance. These flashes of light are caused by reflections from the sun on the large collectors of the iridium satellites.

See also

Literature and Sources

Web links