Daylight Appreciation Day

Daylight Appreciation Day takes place annually on 21. It was created On 21 June 2015 to highlight natural light’s importance to our well-being. Daylight is described as the combination of all direct and indirect sunlight during the daytime. This includes direct sunlight, diffuse sky radiation, and (often) both of these reflected by the Earth and terrestrial objects, like landforms and buildings. Sunlight scattered or reflected by objects in outer space (that is, beyond the Earth’s atmosphere) is generally not considered daylight. Thus, daylight excludes moonlight, despite it being indirect sunlight. Daytime is the period of time each day when daylight occurs. Daylight happens as Earth rotates, and either side on which the Sun shines is considered daylight.

Daylight is present at a particular location, to some degree, whenever the Sun is above the local horizon. (This is true for slightly more than 50% of the Earth at any given time. For an explanation of why it is not exactly half, see here). However, the outdoor illuminance can vary from 120,000 lux for direct sunlight at noon, which may cause eye pain, to less than 5 lux for thick storm clouds with the Sun at the horizon (even <1 lux for the most extreme case), which may make shadows from distant street lights visible. It may be darker under unusual circumstances like a solar eclipse or very high levels of atmospheric particulates, which include smoke and volcanic ash

Daylighting is lighting an indoor space with openings such as windows and skylights that allow daylight into the building. This type of lighting is chosen to save energy, to avoid hypothesized adverse health effects of over-illumination by artificial light, and also for aesthetics. The amount of daylight received into an indoor space or room is defined as a daylight factor, being the ratio between the measured internal and external light levels. Artificial lighting energy use can be reduced by simply installing fewer electric lights because daylight is present, or by dimming/switching electric lights automatically in response to the presence of daylight, a process known as daylight harvesting.

In recent years, work has taken place to recreate the effects of daylight artificially. This is however expensive in terms of both equipment and energy consumption and is applied almost exclusively in specialist areas such as filmmaking, where light of such intensity is required anyway. In some filmmaking locations, such as Sweden or Norway, there is too much light due to long summer days. As a result, in location films such as Marianne (2011), night scenes have to be shot during daylight hours and are digitally altered later.


More International and National Events happening 21 June

Atheist Solidarity Day
Daylight Appreciation Day
Day of the Gong
Go Skateboarding Day
International Yoga Day
National Seashell Day
Peaches ‘N’ Cream Day
World Humanist Day

World Hydrography Day

World Hydrography Day, takes place annually on 21 June. It was adopted by the International Hydrographic Organization as an annual celebration to publicise the work of hydrographers and the importance of hydrography. Hydrography deals with the measurement and description of the physical features of oceans, seas, coastal areas, lakes and rivers, as well as with the prediction of their change over time, for the primary purpose of safety of navigation and in support of all other marine activities, including economic development, security and defence, scientific research, and environmental protection.

The date chosen for World Hydrography Day is the anniversary of the founding of the International Hydrographic Organization which was established in 1921 for the purpose of providing a mechanism for consultation between governments on such matters as technical standards, safe navigation and the protection of the marine environment. The IHO is actively engaged in developing standards and interoperability, particularly in relation to the challenges brought about by digital technologies.

Large scale Hydrography is usually undertaken by national or international organizations which sponsor data collection through precise surveys and publish charts and descriptive material for navigational purposes. The science of oceanography is, in part, an outgrowth of classical hydrography. In many respects the data are interchangeable, but marine hydrographic data will be particularly directed toward marine navigation and safety of that navigation. Marine resource exploration and exploitation is a significant application of hydrography, principally focused on the search for hydrocarbons.

Hydrographical measurements include the tidal, current and wave information of physical oceanography. They include bottom measurements, with particular emphasis on those marine geographical features that pose a hazard to navigation such as rocks, shoals, reefs and other features that obstruct ship passage. Bottom measurements also include collection of the nature of the bottom as it pertains to effective anchoring. Unlike oceanography, hydrography will include shore features, natural and manmade, that aid in navigation. Therefore, a hydrographic survey may include the accurate positions and representations of hills, mountains and even lights and towers that will aid in fixing a ship’s position, as well as the physical aspects of the sea and seabed.

Hydrography charts normally portray what is safe for navigation, and therefore will usually tend to maintain least depths and occasionally de-emphasize the actual submarine topography that would be portrayed on bathymetric charts, which are used to avoid accident. The latter are best representations of the actual seabed, as in a topographic map, for scientific and other purposes. Trends in hydrographic practice since c. 2003–2005 have led to a narrowing of this difference, with many more hydrographic offices maintaining “best observed” databases. This has been coupled with a preference for multi-use surveys, so that the same data collected for nautical charting purposes can also be used for bathymetric portrayal.

Even though, in places, hydrographic survey data may be collected in sufficient detail to portray bottom topography in some areas, hydrographic charts only show depth information relevant for safe navigation and do not show an accurate portrayal of the actual shape of the bottom. The soundings selected from the raw source depth data for placement on the nautical chart are selected for safe navigation and are biased to show predominately the shallowest depths, if there is a deep area that can not be reached because it is surrounded by shallow water, the deep area may not be shown. The color filled areas that show different ranges of shallow water are not the equivalent of contours on a topographic map since they are often drawn seaward of the actual shallowest depth portrayed. A bathymetric chart does show marine topology accurately. Details covering the above limitations can be found in Part 1 of Bowditch’s American Practical Navigator. Another concept that affects safe navigation is the sparsity of detailed depth data from high resolution sonar systems. In more remote areas, the only available depth information has been collected with lead lines. This collection method drops a weighted line to the bottom at intervals and records the depth, often from a rowboat or sail boat. There is no data between soundings or between sounding lines to guarantee that there is not a hazard such as a wreck or a coral head waiting there to ruin a sailor’s day. Often, the navigation of the collecting boat does not match today’s GPS navigational accuracies. The hydrographic chart will use the best data available and will caveat its nature in a caution note or in the legend of the chart.

A hydrographic survey is quite different from a bathymetric survey in some important respects, particularly in a bias toward least depths due to the safety requirements of the former and geomorphologic descriptive requirements of the latter. Historically, this could include echosoundings being conducted under settings biased toward least depths, but in modern practice hydrographic surveys typically attempt to best measure the depths observed, with the adjustments for navigational safety being applied after the fact.

Hydrography of streams includes information on the stream bed, flows, water quality and surrounding land. Basin or interior hydrography pays special attention to rivers and potable water although if collected data is not for ship navigational uses, and is intended for scientific usage, it is more commonly called hydrology. Hydrography of rivers and streams is also an integral part of water management. Most reservoirs in the United States use dedicated stream gauging and rating tables to determine inflows into the reservoir and outflows to irrigation districts, water municipalities and other users of captured water. River/stream hydrographers use handheld and bank mounted devices, to capture a sectional flow rate of moving water through a section.

Each World Hydrography Day has a different theme, these are determined by the Member States of the International Hydrographic Organization in order to promote the importance of hydrography internationally. The theme for World Hydrography Day 2018 is “Bathymetry – the foundation for sustainable seas, oceans and waterways”. Previous themes for World Hydrography day have included Safety at sea and protection of sensitive areas, multilateral cooperation and effective collaboration in data exchange, charting and standards development, Mapping our seas, oceans and waterways, charting our seas and waterways, Hydrography – underpinning the Blue Economy, Hydrography’s importance in managing seas and waterways, supporting safe navigation, Human Resources regarding Hydrography, the Importance of the Hydrographic Services, Hydrography for Protecting the marine environment, The importance of Electronic Navigational Charts (ENCs), safety at sea and efficient maritime practices.

International Yoga Day

The International Day of Yoga (World Yoga Day) is held annually on June 21. It was declared as the International Day of Yoga by the United Nations General Assembly on December 11, 2014. Yoga, is a physical, mental and spiritual practice having its origin over 6000 years ago in India aims to integrate the body and the mind. The declaration of this day came after the call for the adoption of 21 June as International Day of Yoga by Indian Prime Minister Narendra Modi during his address to UN General Assembly on September 27, 2014 wherein he stated:

“Yoga is an invaluable gift of India’s ancient tradition. It embodies unity of mind and body; thought and action; restraint and fulfilment; harmony between man and nature; a holistic approach to health and well-being. It is not about exercise but to discover the sense of oneness within yourself, the world and the nature. By changing our lifestyle and creating consciousness, it can help us deal with climate change. Let us work towards adopting an International Yoga Day.” In suggesting June 21, which is the Summer Solstice, as the International Day of Yoga, Narendra Modi had said that the date is the longest day of the year in the Northern Hemisphere and has special significance in many parts of the world.”

From the perspective of yoga, the Summer Solstice marks the transition to Dakshinayana. The first full moon after Summer Solstice is known as Guru Poornima. According to Sadhguru Jaggi Vasudev, in the yogic lore, the first transmission of yoga by Shiva, the first Guru, is said to have begun on this day. Dakshinayana is also considered a time when there is natural support for those pursuing spiritual practices. Govind Gurbani Yog Guru / Vice President of Rajasthan Swasthya Yog Parisad Jaipur India informed that on 21 June fifty thousand followers of Yoga will perform Yoga in Rajasthan.This initiative found support from many global leaders. At first, the Prime Minister of Nepal Sushil Koirala supported the proposal of Modi. More than 177 countries including USA, Canada, China and Egypt have supported this move, including 175 nations co-sponsoring the resolution. It had the “highest number of co-sponsors ever for any UNGA Resolution of such nature.”On Dec 11, 2014, the 193-member U.N. General Assembly approved by consensus a resolution establishing June 21 as ‘International Day of Yoga’. The resolution also saw a record number of 177 countries co-sponsoring it.

Prior to the 2014 UN resolution declaring June 21 as International Day of Yoga, formal and informal groups of yoga teachers and enthusiasts celebrated World Yoga Day on dates other than 21 June in support of various causes. In December 2011, international humanitarian and meditation and yoga Guru Sri Sri Ravi Shankar and other yoga gurus, supported the cause from the delegation of the Yoga Portuguese Confederation and together gave a call to the UN to declare June 21 as World Yoga Day.

The two-day conference titled ‘Yoga: A Science for World Peace’ was organized by The Art of Living and SVYASA Yoga University, Bengaluru, jointly with Yoga Portuguese Confederation of Lisbon, Portugal on 4th and 5 December 2011, at The Art of living International Centre. On that day, under the leadership of Sri Sri Ravi Shankar a proclamation was signed for having UN and UNESCO declare the 21 June as ‘World Yoga Day’ at Art of Living international Centre at Bangalore, India. HH Sri Sri Ravi Shankar, Founder, The Art of Living; HH Shri Swami Bala Gangadharanath of Adi Chun Chun Giri Mutt; HH Swami Paramátmánanda, General Secretary of the Hindu Dharma Acharya Sabha; BKS Iyengar, Ramamani Iyengar Memorial Yoga Institute, Pune; HH Swami Ramdev, Patañjali Yogpeeth, Haridvar; HH Dr. Nagendra, Vivekánanda Yoga University, Bengaluru; HH Jagat Guru Amrta Súryánanda Mahá Ráj, President of Yoga Portuguese Confederation; HH Subodh Tiwari, Keivalyadhama Yoga Institute; Dr. D.R Kaarthikeyan, Advisor: Law-Human Responsibilities-Corporate Affairs and Dr. Ramesh Bijlani, Aurobindo Ashram, New Delhi were present for the conference.

To Emphasize the importance of yoga, Sri Sri Ravi Shankar said, “Yoga makes you like a child again. When there is yoga and vedanta, there is no lack, impurity, ignorance and injustice. We need to take yoga to the doorstep of everyone and free the world of misery.” According to Jagat Guru Amrta Suryananda, “The idea to celebrate World Yoga Day originated 10 years ago. But this was the first time yoga gurus from India were endorsing it in large numbers, said Jagat Guru Amrta Suryananda Maha Raja, president of the yoga portuguese confederation.,” even before the collective call for such a day by Indian Yoga Gurus in 2011. Following the adoption of the UN Resolution, Sri Sri Ravi Shankar lauded the efforts of Narendra Modi, stating that “It is very difficult for any philosophy, religion or culture to survive without state patronage. Yoga has existed so far almost like an orphan. Now, official recognition by the UN would further spread the benefit of yoga to the entire world.”

Go Skateboarding Day

Go Skateboarding Day (GSD) is an official annual holiday which takes place annually on 21 June. It was conceived by the International Association of Skateboard Companies (IASC) to promote skateboarding. The event was conceived in 2004 by the International Association of Skateboard Companies (IASC) to help make skateboarding more accessible to the world through various events held in major cities around the world. In 2006, more than 350 events took place in 32 countries around the world, and, the following year, the IASC received Special Congressional Recognition from US Congresswoman Loretta Sanchez for its work in promoting the sport of skateboarding and encouraging young people to get outside and enjoy the sport. This holiday is celebrated by millions of skaters around the world. 5boro Skateboards founder/owner, Steve Rodriguez, started Go Skateboarding Day in NYC. It was originally titled the All City Skate Jam. Now GSD accrues thousands of participants in each year all over the world. GSD NYC is considered one of the largest single gatherings of skateboarders in the world and is now considered an official holiday.

In the west country of the UK, Erik Ellington, Spencer Hamilton, Dee Ostrander and Lizard King joined Bristol locals such as Korahn Gayle, Josh Arnott, Paul Carter, and Dave Snaddon to celebrate Go Skateboarding Day in 2014: Thousands of skaters that showed up to Hollenbeck Skate Plaza, LA. North Florida’s Hemming Plaza hosts an annual Go Skate Day celebration.Nike sponsored a large Go Skateboarding Day event in Cologne. The charity organistation Skatistan has been celebrating Go Skateboarding Day in Afghanistan since 2009. In 2014, their events also spread to Cambodia and South Africa, and more than 150 students came along with their families and friends to join street parades in Kabul and over 200 attended celebrations in Mazar-e-Sharif. Vans hold an annual celebration of Go Skate Day in Singapore. In 2014, hundreds of skaters showed up at Sommerset Skate Park for the event. Hundreds of skateboarders joined in to celebrate Go Skateboarding Day in Lima, Peru

Unfortunately Some skaters celebrating Go Skateboarding Day have had conflicts with the law in the past. One of the more notable incidents involved skaters Matt McCormack, Skylar Nalls, Robbie Brindley, Casey Canterbury and several other friends in Hot Springs, Arkansas where they were involved in an incident with police and were arrested. Baird and the involved skaters contend that the police used excessive force, while the police maintain that the officer was enforcing a city ordinance.

World Humanist Day

World Humanist Day is a Humanist holiday celebrated annually around the world on the June solstice, around June 21st As a way of spreading awareness of Humanism as a philosophical life stance and means to effect change in the world. It is also seen as a time for Humanists to gather socially and promote the positive values of Humanism. The holiday developed during the 1980s after several chapters of the American Humanist Association (AHA) began to celebrate it. At the time, the date on which it was celebrated varied from chapter to chapter, with selections such as the founding date of the IHEU, or International Humanist and Ethical Union. From the late 1980s to the early 1990s, the AHA and IHEU passed resolutions declaring World Humanist Day to be on the summer solstice.

Humanism is a philosophical and ethical stance that emphasizes the value and agency of human beings, individually and collectively, and generally prefers critical thinking and evidence (rationalism, empiricism) over acceptance of dogma or superstition. The meaning of the term humanism differs between intellectual movements. The term was coined by theologian Friedrich Niethammer at the beginning of the 19th century. Generally, however, humanism refers to a perspective that affirms some notion of human freedom and progress. In modern times, humanist movements are typically aligned with secularism, and today humanism typically refers to a non-theistic life stance centred on human agency and looking to science rather than revelation from a supernatural source to understand the world

The manner in which World Humanist Day is celebrated varies considerably among local Humanist groups, reflecting the individuality and non-dogmatism of Humanism as a whole. Whilst the event might be a simple gathering, such as a dinner or picnic, with ample time for both socialising and reflection, the method of celebration is down to the individual. Some groups actually develop intricate social rituals, music, and proceedings which highlight the metaphoric symbolism of the solstice and the light (knowledge) which brings us out of darkness (ignorance).

Despite this World Humanist Day has not yet become a fully celebrated holiday in all Humanist organizations, although many of them are beginning to recognize it and plan events and activities around the holiday. International Humanist and Ethical Union lists several different ways that national and local Humanist groups celebrate World Humanist Day. For example, the Dutch Humanist Association broadcast short films about World Humanist Day on Dutch TV in 2012. In 2013, the first National Humanist Day will be organized in The Netherlands. The Humanist Association of Ireland held poetry readings in the park to celebrate the day. The Humanists of Florida Association suggested that groups hold Introduction to Humanism classes on World Humanist Day.

Fete de la Musique

The Fête de la Musique, also known as Make Music Day or World Music Day, is celebrated annually on June 21, around the world. The purpose Of Fête de la Musique is to promote music in two ways with both professional musicians and amateur musicians being encouraged to perform in the streets, under the slogan “Faites de la musique” (“make music”, a homophone of Fête de la Musique).

Music is an important art form and cultural activity whose medium is sound organized in time. The common elements of music are pitch (which governs melody and harmony), rhythm (and its associated concepts tempo, meter, and articulation), dynamics (loudness and softness), and the sonic qualities of timbre and texture (which are sometimes termed the “color” of a musical sound). Different styles or types of music may emphasize, de-emphasize or omit some of these elements. Music is performed with a vast range of instruments and vocal techniques ranging from singing to rapping; there are solely instrumental pieces, solely vocal pieces (such as songs without instrumental accompaniment) and pieces that combine singing and instruments. The word derives from Greek μουσική (mousike; “art of the Muses”). Ancient Greek and Indian philosophers defined music as tones ordered horizontally as melodies and vertically as harmonies. Common sayings such as “the harmony of the spheres” and “it is music to my ears” suggest that music is often ordered and pleasant to listen to. However, 20th-century composer John Cage thought that any sound can be music, saying, for example, “There is no noise, only sound.”

The creation, performance, significance, and even the definition of music vary according to culture and social context. Indeed, throughout history, some new forms or styles of music have been criticized as “not being music”, including Beethoven’s Grosse Fuge string quartet in 1825, early jazz in the beginning of the 1900s and hardcore punk in the 1980s. There are many types of music, including popular music, traditional music, art music, music written for religious ceremonies and work songs such as chanteys. Music ranges from strictly organized compositions–such as Classical music symphonies from the 1700s and 1800s, through to spontaneously played improvisational music such as jazz, and avant-garde styles of chance-based contemporary music from the 20th and 21st centuries.

Music can be divided into genres (e.g., country music) and genres can be further divided into subgenres (e.g., country blues and pop country are two of the many country subgenres), although the dividing lines and relationships between music genres are often subtle, sometimes open to personal interpretation, and occasionally controversial. For example, it can be hard to draw the line between some early 1980s hard rock and heavy metal. Within the arts, music may be classified as a performing art, a fine art or as an auditory art. Music may be played or sung and heard live at a rock concert or orchestra performance, heard live as part of a dramatic work (a music theater show or opera), or it may be recorded and listened to on a radio, MP3 player, CD player, smartphone or as film score or TV show.

In many cultures, music is an important part of people’s way of life, as it plays a key role in religious rituals, rite of passage ceremonies (e.g., graduation and marriage), social activities (e.g., dancing) and cultural activities ranging from amateur karaoke singing to playing in an amateur funk band or singing in a community choir. People may make music as a hobby, like a teen playing cello in a youth orchestra, or work as a professional musician or singer. The music industry includes the individuals who create new songs and musical pieces (such as songwriters and composers), individuals who perform music (which include orchestra, jazz band and rock band musicians, singers and conductors), individuals who record music (music producers and sound engineers), individuals who organize concert tours, and individuals who sell recordings and sheet music and scores to customeors

The concept of an all-day musical celebration on the days of the solstice, was originated by American musician Joel Cohen, who spent two seasons as a producer of musical radio programs for the French National Radio (France Musique). The idea was adapted as a national celebration each June 21 in France and made official by the French Minister of Culture, Jack Lang. The festival later became celebrated in 120 countries around the world.

On October 1981, Maurice Fleuret became Director of Music and Dance at Minister of Culture Jack Lang’s request, and discovered, in a 1982 study on the cultural habits of the French, that five million people, one child out of two, played a musical instrument, he began to dream of a way to bring people out on the streets. It first took place in 1982 in Paris as the Fête de la Musique. Since then he festival has become an international phenomenon, celebrated on the same day in more than 700 cities in 120 countries, including Germany, Italy, Greece, Russia, Australia, Peru, Brazil, Ecuador, Mexico, Canada, the United States, and Japan. Many free concerts are organized during the event, to make all genres of music accessible to the public. in order to organize an official event it must be approved by the official Fête de la Musique organization in Paris and must be free to the public, with all performers donating their time for no fee.

Joey Kramer (Aerosmith)

Happy birthday to Joey Kramer, the American drummer and percussionist with Rock Band Aerosmith, who was Born 21st june 1950. Aerosmith are sometimes referred to as “The Bad Boys from Boston” and “America’s Greatest Rock and Roll Band.” They was formed in Boston, Massachusetts in 1970. Guitarist Joe Perry and bassist Tom Hamilton, originally in a band together called the Jam Band, met up with singer Steven Tyler, drummer Joey Kramer, and guitarist Ray Tabano, and formed Aerosmith.

In 1971, Tabano was replaced by Brad Whitford, and the band began developing a following in Boston, Their style, which is rooted in blues-based hard rock, has come to also incorporate elements of pop, heavy metal, and rhythm and blues, and has inspired many subsequent rock artists. They were signed to Columbia Records in 1972, and released a string of multi-platinum albums, beginning with their 1973 eponymous debut album, followed by their 1974 album Get Your Wings. In 1975, the band broke into the mainstream with the album Toys in the Attic, and their 1976 follow-up Rocks cemented their status as hard rock superstars.The band released two more albums, toured extensively, and charted a string of Hot 100 singles. By the end of the 1970s, they were among the most popular hard rock bands in the world and developed a loyal following of fans, often referred to as the “Blue Army”. However, drug addiction and internal conflict took their toll on the band, which resulted in the departures of Perry and Whitford in 1979 and 1981, respectively; they were replaced by Jimmy Crespo and Rick Dufay. The band did not fare well between 1980 and 1984, releasing a lone album, Rock in a Hard Place, which went gold but failed to match their previous successes.

Perry and Whitford returned in 1984 and the band signed a new deal with Geffen Records. After a comeback tour, the band recorded Done with Mirrors, which won some critical praise but failed to come close to commercial expectations. It was not until the band sobered up and released 1987′s Permanent Vacation that they regained the level of popularity they had experienced in the 1970s. Throughout the late 1980s and 1990s, the band scored several hits including “dude,looks like a lady”and “love in an elvator“, and won numerous awards for music from the multi-platinum albums Pump, Get a Grip, and Nine Lives. The band also became a pop culture phenomenon with popular music videos and notable appearances in television, film, and video games. Their comeback has been described as one of the most remarkable and spectacular in rock ‘n’ roll history. Additional albums followed in 2001 and 2004.

After 42 years of performing, the band continues to tour and record music. The latest album, Music from Another Dimension, was released in, 2012. Aerosmith is the best-selling American rock band of all time, having sold more than 150 million albums worldwide,including 66.5 million albums in the United States alone. They also hold the record for the most gold and multi-platinum albums by an American group. The band has scored 21 Top 40 hits on the Billboard Hot 100, nine number-one Mainstream Rock hits, four Grammy Awards, and ten MTV Video Music Awards. They were inducted into the Rock and Roll Hall of Fame in 2001, and were included among both Rolling Stone’s and VH1′s lists of the 100 Greatest Artists of All Time.

Brandon Flowers (The Killers)

Brandon Flowers, American singer and keyboard player with American rock band The Killers was born June 21st 1981. The killers were formed in 2001, by Brandon Flowers (lead vocals, keyboards) and Dave Keuning (guitar, backing vocals). Mark Stoermer (bass, backing vocals) and Ronnie Vannucci Jr. (drums, percussion) completed the current line-up of the band in 2002.

The name The Killers is derived from a logo on the bass drum of a fictitious band portrayed in the music video for the New Order song “Crystal”.The group has released three studio albums, Hot Fuss (2004), Sam’s Town (2006) and Day & Age (2008)and Battle Born. They have also released one compilation album, Sawdust (2007) and one live album and DVD titled Live from the Royal Albert Hall (2009). To date, the band has sold over 6 million albums in the United States, over 5 million albums in the United Kingdom, and over 15 million worldwide.Some of their best known songs are Mr Brightside,smile like you mean it, When You were Young, Bones, Read my Mind and For Reasons Unknown. The Killers have also performed at T in the Park, Lollapalooza, Glastonbury Festival, V Festival and performed a barnstorming set at the 2013 Isle of Wight Festival

Day and Age: http://m.youtube.com/#/watch?v=2KZPP6Dpnkg
Hot Fuss: http://m.youtube.com/watch?v=_X7q8Bim7X4