Gravity is normally only visible in the ebb and flow of tides. Seawater is continuously exposed to the gravitational force of the moon and the sun, and to centrifugal force owing to the earth's rotation. Luke Jerram's Tide is a sound sculpture that makes these fluctuations in gravity audible. The installation consists of three water-filled glass globes on stands. Tiny changes in gravity measured on the spot cause the water in the globes to rise or fall. When the glass globes are rubbed, they produce a sound akin to that of singing wine glasses: we hear gravity, the variable tide and the interplay between planetary systems.
A gravity meter located in the gallery space, pointed at the center of the earth, measures the changing gravitational pull of the moon and sun on the earth. This information is represented as a video projection showing a full 24 hours of altering gravity. Through the use of water pumps, the received data is also made to control water levels within each sculptural object. A friction device makes the glass of each sculpture resonate and sing (like rubbing a finger around the rim of a wine glass). The rise and fall of water levels over time from high to low tide changes the note produced by each singing sculpture.
Referencing the planets in movement and form, the resonating spheres of glass create a chorus of sounds which fill the gallery space. Being 'directed' live, these machines are altering their state with the changing position of the moon in relation to the gallery.
"During the day, changing gravity causes the sound sculptures to continually change in pitch. But once a day, at precisely the moment of transition from high to low tide (and vice versa), the globes come into harmony. At the magic moment, when the globes are closest to and furthest from the moon, Tide feeds back in an even rhythm. The planetary system of gravitational fields briefly finds a delicate equilibrium and creates a combined spherical sound play." 
This moment of balance, or 'equipoise', is described by Moholy-Nagy in 'The New Vision' (1928). Moholy-Nagy states that, for a sculpture not to be based on illusion, it has to be able to be brought to balanced rest by opposing forces [ebb and tide]. "An actual realization of equipoised sculpture can be made through the application of magnetic forces".  At the time Moholy-Nagy wrote The New Vison he could not have imagined that instead of magnetic or electrical forces, gravitational pull could be used to achieve equipoise.
The final work is based on Kepler's theories of the Music of the Spheres and references early scientific apparatus, as studied in the London Science Museum. The work now functions as both an astronomical clock and a media art exhibit.
“Two years of extensive research was carried out in the development of the work. Advice and support came from over 100 individuals and organizations from around the world including the University of Hawaii Astronomy Department, Medieval musicologists, Clear Night Sky campaigners and a 17th Century glass harmonica maker. NASA provided information on their three-dimensional gravity meter used in submarines for stealth navigation.” 
Relationships with other artworks
The artwork Tide can be can be categorized in artworks that make the viewer aware of environmental changes that can not be observed without special aides. Other notable artworks in this category are Takis’ ‘Signal Lumineux’ (1958), ‘Sun Run Sun’ by Yolande Harris (2008) and Stephen Hurrel’s ‘Beneath And Beyond’ . Just like Tide, Signal Lumineux and Beneath and Beyond transform the slightest changes in the environment to sound and/or motion. Yolande Harris’ Sun Run Sun transforms the movement of nearby satellites into sound with her portable installation. 
 Tide's Official Website, <http://www.lukejerram.com/projects/tide>
 Edward Shanken, red. Art and Electronic Media. London: Phaidon, 2009: p. 193
 AEM Journal Entry for Stephen Hurrel's 'Beneath And Beyond': <http://www.artelectronicmedia.com/?q=node/521>
 AEM Journal Entry for Yolande Harris' 'Sun Run Sun', <http://www.artelectronicmedia.com/?q=node/112>