Waktu Sideris dan Waktu Sinodis

Sidereal time is a measure of the position of the Earth in its rotation around its axis, or time measured by the apparent diurnal motion of the vernal equinox, which is very close to, but not identical to, the motion of stars. They differ by the precession of the vernal equinox in right ascension relative to the stars.

Earth's sidereal day also differs from its rotation period relative to the background stars by the amount of precession in right ascension during one day (8.4 ms). Its J2000 mean value is 23h56m4.090530833s. Etymology of sidereal is from Latin "sidereus" from sidus, sider- = star. Therefore, its meaning relates to a measurement of time relative to the position of the stars.

Definition

Local Sidereal Time (LST) = RA + HA
Greenwich Sidereal Time (GST) is the LST at the Greenwich Meridian

Sidereal time is defined as the hour angle of the vernal equinox. When the meridian of the vernal equinox is directly overhead, local sidereal time is 00:00. Greenwich Sidereal Time is the hour angle of the vernal equinox at the prime meridian at Greenwich, England; local values differ according to longitude. When one moves eastward 15° in longitude, sidereal time is larger by one hour (note that it wraps around at 24 hours). Unlike computing local solar time, differences are counted to the accuracy of measurement, not just in whole hours. Greenwich Sidereal Time and UT1 differ from each other by a constant rate (GST = 1.00273790935 × UT1). Sidereal time is used at astronomical observatories because sidereal time makes it very easy to work out which astronomical objects will be observable at a given time. Objects are located in the night sky using right ascension and declination relative to the celestial equator (analogous to longitude and latitude on Earth), and when sidereal time is equal to an object's right ascension, the object will be at its highest point in the sky, or culmination, at which time it is best placed for observation, as atmospheric extinction is minimized.

Sidereal time and solar time
Solar time is measured by the apparent diurnal motion of the sun, and local noon in solar time is defined as the moment when the sun is at its highest point in the sky (exactly due south or north depending on the observer's latitude and the season). The average time taken for the sun to return to its highest point is 24 hours.

During the time needed by the Earth to complete a rotation around its axis (a sidereal day), the Earth moves a short distance (approximately 1°) along its orbit around the sun. Therefore, after a sidereal day, the Earth still needs to rotate a small extra angular distance before the sun reaches its highest point. A solar day is, therefore, nearly 4 minutes longer than a sidereal day.

The stars, however, are so far away that the Earth's movement along its orbit makes a generally negligible difference to their apparent direction (see, however, parallax), and so they return to their highest point in a sidereal day. A sidereal day is almost 4 minutes shorter than a mean solar day.

Another way to see this difference is to notice that, relative to the stars, the Sun appears to move around the Earth once per year. Therefore, there is one less solar day per year than there are sidereal days. This makes a sidereal day approximately ^365.24⁄_366.24 times the length of the 24-hour solar day, giving approximately 23 hours, 56 minutes, 4.1 seconds (86,164.1 seconds).

Sidereal time vs solar time. Above left: a distant star (the small red circle) and the Sun are at culmination, on the local meridian. Centre: only the distant star is at culmination (a mean sidereal day). Right: few minutes later the Sun is on the local meridian again. A solar day is complete.

Precession effects
The Earth's rotation is not simply a simple rotation around an axis that would always remain parallel to itself. The Earth's rotational axis itself rotates about a second axis, orthogonal to the Earth's orbit, taking about 25,800 years to perform a complete rotation. This phenomenon is called the precession of the equinoxes. Because of this precession, the stars appear to move around the Earth in a manner more complicated than a simple constant rotation.

For this reason, to simplify the description of Earth's orientation in astronomy and geodesy, it is conventional to describe Earth's rotation relative to a frame which is itself precessing slowly. In this reference frame, Earth's rotation is close to constant, but the stars appear to rotate slowly with a period of about 25,800 years. It is also in this reference frame that the tropical year, the year related to the Earth's seasons, represents one orbit of the Earth around the sun. The precise definition of a sidereal day is the time taken for one rotation of the Earth in this precessing reference frame.

Exact duration and its variation

A mean sidereal day is about 23 h 56 m 4.1 s in length. However, due to variations in the rotation rate of the Earth the rate of an ideal sidereal clock deviates from any simple multiple of a civil clock. In practice, the difference is kept track of by the difference UTC–UT1, which is measured by radio telescopes and kept on file and available to the public at the IERS and at the United States Naval Observatory.

Given a tropical year of 365.242190402 days from Simon et al. this gives a sidereal day of 86,400 × ^{365.242190402}⁄_{366.242190402}, or 86,164.09053 seconds.

According to Aoki et al., an accurate value for the sidereal day at the beginning of 2000 is ¹⁄_{1.002737909350795} times a mean solar day of 86,400 seconds, which gives 86,164.090530833 seconds. For times within a century of 1984, the ratio only alters in its 11th decimal place. This web-based sidereal time calculator uses a truncated ratio of ¹⁄_{1.00273790935}.

Because this is the period of rotation in a precessing reference frame, it is not directly related to the mean rotation rate of the Earth in an inertial frame, which is given by ω=2π/T where T is the slightly longer stellar day given by Aoki et al. as 86,164.09890369732 seconds. This can be calculated by noting that ω is the magnitude of the vector sum of the rotations leading to the sidereal day and the precession of that rotation vector. In fact, the period of the Earth's rotation varies on hourly to interannual timescales by around a millisecond, together with a secular increase in length of day of about 2.3 milliseconds per century which mostly results from slowing of the Earth's rotation by tidal friction.

Source : wikipedia, Polaris Project,Positional Astronomy

Example:

Teleskop Hubble Menunjukkan Bukti Keberadaan Dark Matter di sekitar galaksi kecil

Hubble Space Telescope (HST) milik NASA/ESA telah menemukan serangkaian bukti kuat tentang keberadaan dark matter di halo galaksi.

HST mengamati kluster galaksi Perseus dan menemukan banyak galaksi kecil yang tetap utuh sementara galaksi besar di sekitarnya "terkoyak" oleh gaya gravitasi satu sama lain.

Gambar yang diperoleh HST menunjukkan bukti lebih lanjut bahwa galaksi kecil tersebut dikelilingi oleh dark matter yang melindunginya dari gaya gravitasi galaksi tetangga yang bisa menghancurkannya.

Dark matter adalah sejenis materi yang tidak memancarkan radiasi (sehingga dianggap "dark" karena tidak teramati secara visual) tetapi dapat dideteksi dari efek gaya gravitasi yang ditimbulkan pada objek yang teramati, seperti bintang, gas dan debu, dll. Dark matter mendominasi materi yang ada di alam semesta ini.

Astronom Christopher Conselice dari Universitas Nottingham, UK mengatakan bahwa timnya (beliau memimpin tim pengamatan dengan HST) terkejut ketika menemukan begitu banyak galaksi kerdil (dwarf galaxy) di bagian inti kluster ini (Perseus, red) yang begitu mulus dengan bentuk bulat dan tidak ada tanda-tanda adanya gangguan gravitasi dari galaksi lain. Galaksi kerdil tersebut adalah galaksi yang sudah sangat tua sehingga jika ada sesuatu yang bisa menghancurkannya (gaya gravitasi galaksi lain, misalnya) pasti sudah akan terjadi saat ini. Galaksi - galaksi ini pastilah didominasi oleh dark matter.

Galaksi kerdil tersebut mungkin mengandung dark matter lebih banyak daripada galaksi spiral. Hal ini disimpulkan berdasarkan pengamatan bahwa galaksi spiral di kluster Perseus hancur (bentuknya terdistorsi) sedangkan galaksi kerdil tersebut tidak. Meskipun begitu, Concelice tidak dapat mengatakan bahwa kandungan dark matter di galaksi kerdil ini lebih tinggi dibandingkan galaksi kita, BimaSakti (Milky Way).

Diajukan pertama kali oleh astronom Swiss Fritz Zwicky, dark matter dianalogikan seperti lem yang mengikat galaksi - galaksi. Astronom percaya bahwa dark matter mempunyai perna penting dalam pembentukan galaksi lewat tarikan gravitasinya.

Observasi dengan Hubble's Advanced Camera for Surveys menemukan 29 galaksi kerdil elips di Kluster Perseus yang berjarak 250 juta tahun cahaya dan merupakan salah satu kluster galaksi terdekat.

Karena dark matter tidak dapat "dilihat", astronom mendeteksi keberadaannya dengan pengukuran tidak langsung. Metode yang umum adalah dengan mengukur kecepatan dari masing-masing bintang atau kelompok bintang seiring gerakan acaknya atau gerakannya mengitari galaksi.

Kluster Perseus terlalu jauh bagi teleskop untuk mengamati bintang tunggal dan mengukur kecepatan geraknya. Jadi, Conselice dan timnya menemukan teknik baru untuk menemukan dark matter di galaksi kerdil ini dengan menentukan kontribusi massa tambahan minimum dari dark matter yang harus dipunyai galaksi kerdil untuk melindunginya dari kehancuran akibat gaya tarik gravitasi dari galaksi lain yang lebih besar ukurannya.

Dengan mempelajari galaksi - galaksi kerdil ini secara detail hanya dimungkinkan oleh ketajaman resolusi Hubble's Advanced Camera for Surveys. Conselice dan timnya pertama kali mengamati kluster Perseus dengan teleskop WIYN di Kitt Peak National Observatory (dipunyai dan dioperasikan oleh konsorsium WIYN, yang terdiri dari University of Wisconsin, Indiana University, Yale University, dan National Optical Astronomy Observatory). Hasil pengamatan yang diperoleh dengan teleskop tersebut hanya mengindikasikan bahwa galaksi - galaksi yang diamati Concelice dan timnya sangat "smooth" dan didominasi oleh dark matter tetapi teleskop tersebut tidak cukup resolusinya untuk melakukan pengamatan secara detail. Oleh sebab itu, Concelice dan timnya menggunakan HST untuk melakukan pengamatan ini.

Sumber : ESA

Trivia : Two Objects

Can you mention the name of these two objects (see the picture above)? Are they included in Messier Catalog's objects? If yes, mention its Messier number.

You can submit your answer and discuss with other reader in the comment section.

GLOBE at Night 2009 - Can You See the Stars?

Turning out the lights for "Earth Hour" is going to be a great way to demonstrate caring about climate changes by turning off the lights - but what about the impact that light pollution has on our skies? 2008 marked a monumental shift in human history when the number of people living in cities exceeded half the people on Earth. Because of the ambient light of urban landscapes, many city dwellers have never seen a sky full of stars. Are you interested in helping science study the impact of lighting in your area? Then step inside and learn more about GLOBE…

GLOBE at Night is a wonderful way for everyone around the world to participate as a citizen-sciencist to raise public awareness of the impact of artificial lighting on local environments. This event encourages everyone - students, educators, dark sky advocates and the general public - to measure the darkness of their local skies and contribute their observations online to a world map. GLOBE at Night is a centerpiece of the Dark Skies Awareness Global Cornerstone Project for the International Year of Astronomy (IYA) in 2009, and we need people - just like you - to get involved! Data collection and online reporting is simple and user-friendly.

Led by the educational outreach staff at the National Optical Astronomy Observatory and the University Corporation for Atmospheric Research GLOBE Program, the GLOBE at Night campaign will take place for a 4th year from March 16-28, 2009. “The geographic reach of the GLOBE at Night program exceeded our wildest expectations,” said Connie Walker, an astronomer and science education specialist at the National Optical Astronomy Observatory (NOAO), one of the event’s major co-sponsors. “We fell a few hundred short of our target of 5,000 total observations, but the engagement and excitement of large family groups, and dozens of school children participating in the activity together, more than make up for a few less data points.”

Over the past 3 years, tens of thousands of citizen-scientists around the world have contributed measurements of their local sky brightness to a growing global database in two ways: simple unaided-eye observations toward the constellation Orion and quantative digital measurements through a handheld, well-calibrated sky-brightness meter. For the first method, citizen-scientists take data on light pollution levels by comparing what they see toward Orion, with star maps showing different stellar brightness limits. The basic idea is to look for the faintest stars and match them to one of seven star maps of progressively fainter limiting magnitudes. For the second method, digital sky-brightness meters are used for more precise measurements. The low-cost digital Sky Quality Meters (SQMs), manufactured by Unihedron, can make a highly repeatable, direct measurement of integrated sky brightness. The newly available second-generation of SQM-Ls being used this year by several GLOBE at Night sites has a cone-shaped “field of view” that is three times more narrow than the older model. This specifically aids its use in city environments, where surrounding lights or buildings may affect the readings.

To learn the five easy steps to participate in either type of GLOBE at Night program and to obtain important information on light pollution, stellar magnitudes, the mythology of Orion, how to find Orion, how to obtain your latitude and longitude, and how to use an SQM, see the GLOBE at Night website. No prior experience is necessary and all the information you need to participate is right there - along with downloads for activity kits for families, teachers and invididuals in six different languages. All observations will be available online via Google Earth and as downloadable datasets, too.


Thanks to an international network of partners, GLOBE reaches people around the world, and during their first two years managed 20,000 observations from a total of 100 countries. This year, they're hoping for an even greater success rate and within weeks of submitting your data, a world map showing the results of your studies will be made available. Using this information, you can then compare the data to previous studies, as well as satellite data and population density data. Collecting information from mulitple locations inside a single city or region is highly encourged, and would make a great class project or astronomy club activity!

By activity participating in projects like GLOBE, you can make difference. More measurements made each year and over the next few years will allow for in more depth analysis. More measurements within a city will provide maps of higher resolution and comparisons between years would allow people to monitor changes. Just like our other Earthly environments, monitoring our lighting environment will allow us as citizen-scientists to identify and preserve dark sky locations in cities or catch an area developing too quickly and influence people to make smart choices in lighting by providing them with informed neighbors. As just everyday, ordinary people, we can impact what happens by educating ourselves and others. If more and more people took a few minutes during the March 2009 campaign to measure sky brightness either toward Orion with the unaided-eye or toward zenith with a Sky Quality Meter (or both!), their measurements - and yours - will make a world of difference!

Cited from : Universe Today - Tammy Plotner

Article : Watching Venus Glow In The Dark

Atmospheric investigations by Venus Express

ESA’s Venus Express spacecraft has observed an eerie glow in the night-time atmosphere of Venus. This infrared light comes from nitric oxide and is showing scientists that the atmosphere of Earth’s nearest neighbour is a temperamental place of high winds and turbulence.

Unfortunately, the glow on Venus cannot be seen with the naked eye because it occurs at the invisible wavelengths of infrared. ESA’s Venus Express, however, is equipped with the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument, which can see these wavelengths.

VIRTIS has made two unambiguous detections of the so-called nightglow for nitric oxide at Venus. This is the first time such infrared detections have been made for any planet and provide a new insight into Venus’s atmosphere.

“The nightglow can give us a lot of information,” says Antonio García Muñoz, who was at the Australian National University when the research was carried out; he is now located at the Instituto de Astrofísica de Canarias, Tenerife, Spain. "It can provide details about the temperature, wind direction, composition and chemistry of an atmosphere."

The nightglow is ultimately caused by the Sun’s ultraviolet light, which streams into a planet’s atmosphere and breaks the molecules up into atoms and other simpler molecules. The free atoms may recombine again and, in specific cases, the resulting molecule is endowed with some extra energy that is subsequently lost in the form of light. On the day-side of the planet, any atoms that do find their way back together are outshone by the sunlight falling into the atmosphere.

But on the night-side, where atoms are transported by a vigorous diurnal circulation, the glow can be seen with appropriate instruments, such as VIRTIS.

A nitric oxide nightglow in the infrared has never been observed in the atmospheres of Mars or Earth, although we know that the necessary nitric oxide molecules are present because they have been observed in ultraviolet.

Nightglow in Venus’s atmosphere

The nightglow on Venus has been seen at infrared wavelengths before, betraying oxygen molecules and the hydroxyl radical, but this is the first detection of nitric oxide at those wavelengths. It offers data about the atmosphere of Venus that lies above the cloud tops at around 70 km. The oxygen and hydroxyl emissions come from 90-100 km, whereas the nitric oxide comes from 110-120 km altitude.

Yet, even VIRTIS cannot see the nitric oxide nightglow all the time because it is often just too faint. “Luckily for us, Venus has a temperamental atmosphere,” says García Muñoz, “Packets of oxygen and nitrogen atoms are blown around.” Sometimes these become dense enough to boost the brightness of the nightglow, making it visible to VIRTIS.

Venus Express can observe the three nightglow emissions simultaneously, and this gives rise to a mystery. The nightglows from the different molecules do not necessarily happen together. “Perhaps when we have more observations, we will understand the correlation between them,” says García Muñoz.

In order to do that, the VIRTIS team plans to continue monitoring the planet, building up a database of this fascinating phenomenon.

Cited from : ESA

Trivia Question : Guess The Name Of This Object

It's been long time since the last time I posted a trivia question. This one is an easy one. Can you mention the name of the object?